$ES
Eversource Energy Stock $es

$es Overview Financials Valuation SEC Filings Company Website

2012 Q3 Form 10-K Financial Statement

#000007274113000015 Filed on February 28, 2013

Income Statement

Concept	2012 Q3	2012 Q2	2012
Revenue	$1.862B	$1.629B	$6.274B
YoY Change	66.97%	55.48%	40.49%
Cost Of Revenue
YoY Change
Gross Profit
YoY Change
Gross Profit Margin
Selling, General & Admin
YoY Change
% of Gross Profit
Research & Development
YoY Change
% of Gross Profit
Depreciation & Amortization	$231.3M	$144.5M	$740.8M
YoY Change	79.3%	33.78%	59.93%
% of Gross Profit
Operating Expenses	$1.449B	$1.469B	$5.156B
YoY Change	59.0%	69.0%	40.42%
Operating Profit	$412.9M	$159.5M	$1.118B
YoY Change	102.6%	-10.44%	40.8%
Interest Expense	$90.36M	-$89.00M	$329.9M
YoY Change	41.34%	-243.01%	31.75%
% of Operating Profit	21.88%	-55.8%	29.51%
Other Income/Expense, Net	$4.324M	$1.806M	$19.74M
YoY Change	202.38%	-75.37%	-28.77%
Pretax Income	$326.9M	$72.30M	$808.0M
YoY Change	131.31%	-41.33%	41.39%
Income Tax	$117.4M	$26.06M	$274.9M
% Of Pretax Income	35.9%	36.04%	34.03%
Net Earnings	$209.5M	$46.20M	$533.1M
YoY Change	129.21%	-41.3%	33.1%
Net Earnings / Revenue	11.25%	2.84%	8.5%
Basic Earnings Per Share	$0.66	$0.15	$1.90
Diluted Earnings Per Share	$0.66	$0.15	$1.89
COMMON SHARES
Basic Shares Outstanding	314.8M shares	301.0M shares	277.2M shares
Diluted Shares Outstanding	315.8M shares	301.8M shares	278.0M shares

Balance Sheet

Concept	2012 Q3	2012 Q2	2012
SHORT-TERM ASSETS
Cash & Short-Term Investments	$155.0M	$108.0M	$46.00M
YoY Change	70.33%	20.0%	-41.03%
Cash & Equivalents	$73.00M	$28.00M	$46.00M
Short-Term Investments	$82.00M	$79.00M
Other Short-Term Assets	$113.0M	$922.0M	$1.121B
YoY Change	-13.11%	815.02%	106.45%
Inventory	$267.0M	$263.0M	$268.0M
Prepaid Expenses
Receivables	$773.4M	$662.0M	$793.0M
Other Receivables	$0.00	$0.00	$0.00
Total Short-Term Assets	$2.139B	$1.954B	$2.227B
YoY Change	61.14%	46.05%	64.11%
LONG-TERM ASSETS
Property, Plant & Equipment	$16.30B	$16.06B	$16.61B
YoY Change	61.49%	62.76%	59.62%
Goodwill	$3.518B
YoY Change	1123.42%
Intangibles
YoY Change
Long-Term Investments	$394.0M	$372.0M	$400.0M
YoY Change	657.69%	541.38%	566.67%
Other Assets	$300.8M	$5.607B	$5.551B
YoY Change	79.03%	3585.74%	56.85%
Total Long-Term Assets	$9.315B	$25.55B	$26.08B
YoY Change	181.81%	688.47%	82.48%
TOTAL ASSETS
Total Short-Term Assets	$2.139B	$1.954B	$2.227B
Total Long-Term Assets	$9.315B	$25.55B	$26.08B
Total Assets	$27.76B	$27.51B	$28.30B
YoY Change	88.46%	90.45%	80.88%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$542.7M	$601.0M	$764.0M
YoY Change	13.96%	60.78%	20.7%
Accrued Expenses
YoY Change
Deferred Revenue
YoY Change
Short-Term Debt	$1.359B	$1.249B	$1.120B
YoY Change	4430.0%	811.68%	253.31%
Long-Term Debt Due	$879.7M	$683.0M	$763.0M
YoY Change	163.14%	102.68%	129.82%
Total Short-Term Liabilities	$3.801B	$3.490B	$3.644B
YoY Change	147.55%	136.45%	87.06%
LONG-TERM LIABILITIES
Long-Term Debt	$6.733B	$7.097B	$7.282B
YoY Change	45.89%	62.92%	54.05%
Other Long-Term Liabilities	$871.4M	$4.447B	$4.521B
YoY Change	29.86%	538.9%	52.12%
Total Long-Term Liabilities	$7.791B	$11.54B	$11.80B
YoY Change	78.13%	160.6%	53.31%
TOTAL LIABILITIES
Total Short-Term Liabilities	$3.801B	$3.490B	$3.644B
Total Long-Term Liabilities	$7.791B	$11.54B	$11.80B
Total Liabilities	$11.59B	$18.28B	$18.91B
YoY Change	96.17%	209.57%	64.18%
SHAREHOLDERS EQUITY
Retained Earnings	$1.736B
YoY Change	9.33%
Common Stock	$1.662B
YoY Change	69.62%
Preferred Stock
YoY Change
Treasury Stock (at cost)	-$340.7M
YoY Change	-2.32%
Treasury Stock Shares	18.57M shares
Shareholders Equity	$9.177B	$9.223B	$9.393B
YoY Change
Total Liabilities & Shareholders Equity	$27.76B	$27.51B	$28.30B
YoY Change	88.46%	90.45%	80.88%

Cashflow Statement

Concept	2012 Q3	2012 Q2	2012
OPERATING ACTIVITIES
Net Income	$209.5M	$46.20M	$533.1M
YoY Change	129.21%	-41.3%	33.1%
Depreciation, Depletion And Amortization	$231.3M	$144.5M	$740.8M
YoY Change	79.3%	33.78%	59.93%
Cash From Operating Activities	$475.6M	$311.6M	$1.161B
YoY Change	135.8%	-0.73%	19.67%
INVESTING ACTIVITIES
Capital Expenditures	-$391.4M	-$386.1M	$1.472B
YoY Change	39.49%	66.57%	36.74%
Acquisitions
YoY Change
Other Investing Activities	$19.80M	-$1.200M	-$35.68M
YoY Change	-1337.5%	-102.92%	-41.19%
Cash From Investing Activities	-$371.6M	-$387.2M	-$1.468B
YoY Change	31.68%	102.94%	44.11%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net			$839.1M
YoY Change			127.05%
Cash From Financing Activities	-59.10M	-179.2M	$345.9M
YoY Change	-172.07%	45.22%	1002.77%
NET CHANGE
Cash From Operating Activities	475.6M	311.6M	$1.161B
Cash From Investing Activities	-371.6M	-387.2M	-$1.468B
Cash From Financing Activities	-59.10M	-179.2M	$345.9M
Net Change In Cash	44.90M	-254.8M	$39.19M
YoY Change	2893.33%	84833.33%	-332.77%
FREE CASH FLOW
Cash From Operating Activities	$475.6M	$311.6M	$1.161B
Capital Expenditures	-$391.4M	-$386.1M	$1.472B
Free Cash Flow	$867.0M	$697.7M	-$311.0M
YoY Change	79.76%	27.85%	192.49%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2010	nu	Peaker Cfd Expense Incurred PeakerCfdExpenseIncurred	10000000	USD
CY2012	nu	Fuel Costs Incurred FuelCostsIncurred	105200000	USD
CY2010	nu	Renewable Energy Costs Incurred RenewableEnergyCostsIncurred	5800000	USD
CY2012Q3	us-gaap	Net Income Loss NetIncomeLoss	209500000	USD
CY2011Q4	us-gaap	Treasury Stock Value TreasuryStockValue	-346667000	USD
CY2011	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	394693000	USD
CY2012	us-gaap	Repayments Of Long Term Debt RepaymentsOfLongTermDebt	839136000	USD
CY2010	nu	Natural Gas Procurement Costs Incurred NaturalGasProcurementCostsIncurred	209500000	USD
CY2012Q4	us-gaap	Additional Paid In Capital AdditionalPaidInCapital	6183267000	USD
CY2011Q4	us-gaap	Additional Paid In Capital AdditionalPaidInCapital	1797884000	USD
CY2012Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	1802714000	USD
CY2011Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	1651875000	USD
CY2012Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	72854000	USD
CY2011Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	70686000	USD
CY2012Q4	us-gaap	Treasury Stock Value TreasuryStockValue	-338624000	USD
CY2010	nu	Transmission Support Commitment Costs Incurred TransmissionSupportCommitmentCostsIncurred	18900000	USD
CY2012Q3	us-gaap	Revenues Revenues	1861500000	USD
CY2012Q3	us-gaap	Operating Income Loss OperatingIncomeLoss	412900000	USD
CY2012Q2	us-gaap	Operating Income Loss OperatingIncomeLoss	159500000	USD
CY2012Q4	us-gaap	Other Assets Current OtherAssetsCurrent	107972000	USD
CY2012Q4	us-gaap	Assets Current AssetsCurrent	2227295000	USD
CY2012Q4	us-gaap	Cash Cash	45748000	USD
CY2012Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	792822000	USD
CY2012Q4	us-gaap	Costs In Excess Of Billings On Uncompleted Contracts Or Programs Expected To Be Collected Within One Year CostsInExcessOfBillingsOnUncompletedContractsOrProgramsExpectedToBeCollectedWithinOneYear	216040000	USD
CY2012Q4	us-gaap	Energy Related Inventory EnergyRelatedInventory	267713000	USD
CY2012Q4	us-gaap	Marketable Securities Current MarketableSecuritiesCurrent	91975000	USD
CY2010	us-gaap	Amortization Of Regulatory Asset AmortizationOfRegulatoryAsset	90054000	USD
CY2010	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	387949000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-27316000	USD
CY2011	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	367377000	USD
CY2011	us-gaap	Other Depreciation And Amortization OtherDepreciationAndAmortization	69912000	USD
CY2012	us-gaap	Taxes Excluding Income And Excise Taxes TaxesExcludingIncomeAndExciseTaxes	434207000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-2168000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	97000	USD
CY2012	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	7132000	USD
CY2011Q4	us-gaap	Cash Cash	6559000	USD
CY2011Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	488002000	USD
CY2011Q4	us-gaap	Costs In Excess Of Billings On Uncompleted Contracts Or Programs Expected To Be Collected Within One Year CostsInExcessOfBillingsOnUncompletedContractsOrProgramsExpectedToBeCollectedWithinOneYear	175207000	USD
CY2011Q4	us-gaap	Energy Related Inventory EnergyRelatedInventory	248958000	USD
CY2011Q4	us-gaap	Marketable Securities Current MarketableSecuritiesCurrent	70970000	USD
CY2011Q4	us-gaap	Other Assets Current OtherAssetsCurrent	112632000	USD
CY2011Q4	us-gaap	Assets Current AssetsCurrent	1357472000	USD
CY2012Q4	us-gaap	Regulatory Assets Noncurrent RegulatoryAssetsNoncurrent	5132411000	USD
CY2011Q4	us-gaap	Regulatory Assets Noncurrent RegulatoryAssetsNoncurrent	3267710000	USD
CY2012Q4	us-gaap	Goodwill Goodwill	3519401000	USD
CY2011Q4	us-gaap	Goodwill Goodwill	287591000	USD
CY2012Q4	us-gaap	Marketable Securities Noncurrent MarketableSecuritiesNoncurrent	400329000	USD
CY2011Q4	us-gaap	Marketable Securities Noncurrent MarketableSecuritiesNoncurrent	60311000	USD
CY2012Q4	us-gaap	Derivative Assets Noncurrent DerivativeAssetsNoncurrent	90612000	USD
CY2011Q4	us-gaap	Derivative Assets Noncurrent DerivativeAssetsNoncurrent	98357000	USD
CY2012Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	327766000	USD
CY2011Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	172560000	USD
CY2012Q4	us-gaap	Assets Noncurrent AssetsNoncurrent	9470519000	USD
CY2011Q4	us-gaap	Assets Noncurrent AssetsNoncurrent	3886529000	USD
CY2012Q4	us-gaap	Assets Assets	28302824000	USD
CY2012Q4	us-gaap	Regulatory Assets Current RegulatoryAssetsCurrent	705025000	USD
CY2011Q4	us-gaap	Regulatory Assets Current RegulatoryAssetsCurrent	255144000	USD
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Net PublicUtilitiesPropertyPlantAndEquipmentNet	16605010000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Net PublicUtilitiesPropertyPlantAndEquipmentNet	10403065000	USD
CY2012Q4	us-gaap	Notes Payable To Bank Current NotesPayableToBankCurrent	1120196000	USD
CY2011Q4	us-gaap	Notes Payable To Bank Current NotesPayableToBankCurrent	317000000	USD
CY2012Q4	us-gaap	Long Term Debt Current LongTermDebtCurrent	763338000	USD
CY2011Q4	us-gaap	Long Term Debt Current LongTermDebtCurrent	331582000	USD
CY2012Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	764350000	USD
CY2011Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	633282000	USD
CY2012Q4	us-gaap	Derivative Liabilities Current DerivativeLiabilitiesCurrent	117194000	USD
CY2011Q4	us-gaap	Derivative Liabilities Current DerivativeLiabilitiesCurrent	107558000	USD
CY2012Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	744497000	USD
CY2011Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	390416000	USD
CY2012Q4	us-gaap	Liabilities Current LiabilitiesCurrent	3643690000	USD
CY2011Q4	us-gaap	Liabilities Current LiabilitiesCurrent	1947682000	USD
CY2012Q4	us-gaap	Secured Long Term Debt SecuredLongTermDebt	82139000	USD
CY2011Q4	us-gaap	Secured Long Term Debt SecuredLongTermDebt	112260000	USD
CY2012Q4	us-gaap	Deferred Tax Liabilities Noncurrent DeferredTaxLiabilitiesNoncurrent	3463347000	USD
CY2011Q4	us-gaap	Deferred Tax Liabilities Noncurrent DeferredTaxLiabilitiesNoncurrent	1868316000	USD
CY2012Q4	us-gaap	Regulatory Liability Noncurrent RegulatoryLiabilityNoncurrent	540162000	USD
CY2011Q4	us-gaap	Regulatory Liability Noncurrent RegulatoryLiabilityNoncurrent	266145000	USD
CY2012Q4	us-gaap	Derivative Liabilities Noncurrent DerivativeLiabilitiesNoncurrent	882654000	USD
CY2011Q4	us-gaap	Derivative Liabilities Noncurrent DerivativeLiabilitiesNoncurrent	959876000	USD
CY2012Q4	us-gaap	Pension And Other Postretirement Defined Benefit Plans Liabilities Noncurrent PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent	2130497000	USD
CY2011Q4	us-gaap	Pension And Other Postretirement Defined Benefit Plans Liabilities Noncurrent PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent	1326037000	USD
CY2012Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	967561000	USD
CY2011Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	420011000	USD
CY2012Q4	us-gaap	Liabilities Noncurrent LiabilitiesNoncurrent	7984221000	USD
CY2011Q4	us-gaap	Liabilities Noncurrent LiabilitiesNoncurrent	4840385000	USD
CY2012Q4	us-gaap	Stockholders Equity StockholdersEquity	9237050000	USD
CY2011Q4	us-gaap	Stockholders Equity StockholdersEquity	4012670000	USD
CY2012Q4	us-gaap	Long Term Debt Noncurrent LongTermDebtNoncurrent	7200156000	USD
CY2011Q4	us-gaap	Long Term Debt Noncurrent LongTermDebtNoncurrent	4614913000	USD
CY2012Q4	us-gaap	Temporary Equity Carrying Amount Attributable To Noncontrolling Interest TemporaryEquityCarryingAmountAttributableToNoncontrollingInterest	155568000	USD
CY2011Q4	us-gaap	Temporary Equity Carrying Amount Attributable To Noncontrolling Interest TemporaryEquityCarryingAmountAttributableToNoncontrollingInterest	116200000	USD
CY2012Q4	us-gaap	Common Stock Value CommonStockValue	1662547000	USD
CY2011Q4	us-gaap	Common Stock Value CommonStockValue	980264000	USD
CY2011	us-gaap	Operating Income Loss OperatingIncomeLoss	794176000	USD
CY2010	us-gaap	Other Depreciation And Amortization OtherDepreciationAndAmortization	232871000	USD
CY2011	us-gaap	Taxes Excluding Income And Excise Taxes TaxesExcludingIncomeAndExciseTaxes	323610000	USD
CY2011	us-gaap	Utilities Operating Expense Maintenance And Operations UtilitiesOperatingExpenseMaintenanceAndOperations	1095358000	USD
CY2010	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	6158000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Net Of Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax	200000	USD
CY2012	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	523777000	USD
CY2011Q4	us-gaap	Assets Assets	15647066000	USD
CY2010	us-gaap	Taxes Excluding Income And Excise Taxes TaxesExcludingIncomeAndExciseTaxes	314741000	USD
CY2012	us-gaap	Operating Expenses OperatingExpenses	5155581000	USD
CY2010	us-gaap	Revenues Revenues	4898167000	USD
CY2012	us-gaap	Cost Of Purchased Power CostOfPurchasedPower	2084364000	USD
CY2011	us-gaap	Cost Of Purchased Power CostOfPurchasedPower	1657914000	USD
CY2010	us-gaap	Cost Of Purchased Power CostOfPurchasedPower	2034501000	USD
CY2012Q4	us-gaap	Minority Interest MinorityInterest	0	USD
CY2011Q4	us-gaap	Minority Interest MinorityInterest	2956000	USD
CY2012Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	9237050000	USD
CY2011Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	4015626000	USD
CY2012Q4	us-gaap	Capitalization Longterm Debt And Equity CapitalizationLongtermDebtAndEquity	16592774000	USD
CY2011Q4	us-gaap	Capitalization Longterm Debt And Equity CapitalizationLongtermDebtAndEquity	8746739000	USD
CY2012Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	28302824000	USD
CY2011Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	15647066000	USD
CY2012Q4	us-gaap	Regulatory Liability Current RegulatoryLiabilityCurrent	134115000	USD
CY2011Q4	us-gaap	Regulatory Liability Current RegulatoryLiabilityCurrent	167844000	USD
CY2012	us-gaap	Revenues Revenues	6273787000	USD
CY2011	us-gaap	Revenues Revenues	4465657000	USD
CY2012	us-gaap	Depreciation Depreciation	519010000	USD
CY2011	us-gaap	Depreciation Depreciation	302192000	USD
CY2010	us-gaap	Depreciation Depreciation	300737000	USD
CY2012	us-gaap	Amortization Of Regulatory Asset AmortizationOfRegulatoryAsset	79762000	USD
CY2011	us-gaap	Amortization Of Regulatory Asset AmortizationOfRegulatoryAsset	91080000	USD
CY2012	us-gaap	Other Depreciation And Amortization OtherDepreciationAndAmortization	142019000	USD
CY2011	us-gaap	Operating Expenses OperatingExpenses	3671481000	USD
CY2010	us-gaap	Operating Expenses OperatingExpenses	4098276000	USD
CY2012	us-gaap	Operating Income Loss OperatingIncomeLoss	1118206000	USD
CY2010	us-gaap	Operating Income Loss OperatingIncomeLoss	799891000	USD
CY2012	us-gaap	Interest Expense Long Term Debt InterestExpenseLongTermDebt	316987000	USD
CY2011	us-gaap	Interest Expense Long Term Debt InterestExpenseLongTermDebt	231630000	USD
CY2010	us-gaap	Interest Expense Long Term Debt InterestExpenseLongTermDebt	231089000	USD
CY2012	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	6168000	USD
CY2011	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	8611000	USD
CY2010	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	20573000	USD
CY2012	us-gaap	Interest Expense Other InterestExpenseOther	6790000	USD
CY2011	us-gaap	Interest Expense Other InterestExpenseOther	10184000	USD
CY2010	us-gaap	Interest Expense Other InterestExpenseOther	-14371000	USD
CY2012	us-gaap	Interest Expense InterestExpense	329945000	USD
CY2011	us-gaap	Interest Expense InterestExpense	250425000	USD
CY2010	us-gaap	Interest Expense InterestExpense	237291000	USD
CY2012	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	19742000	USD
CY2011	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	27715000	USD
CY2010	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	41916000	USD
CY2012	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	808003000	USD
CY2011	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	571466000	USD
CY2010	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	604516000	USD
CY2012	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	274926000	USD
CY2011	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	170953000	USD
CY2010	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	210409000	USD
CY2012	us-gaap	Net Income Loss NetIncomeLoss	533077000	USD
CY2011	us-gaap	Net Income Loss NetIncomeLoss	400513000	USD
CY2010	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	388046000	USD
CY2012	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	-34321000	USD
CY2012	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	292000000	USD
CY2010	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	210939000	USD
CY2012	us-gaap	Pension And Other Postretirement Benefit Expense PensionAndOtherPostretirementBenefitExpense	218540000	USD
CY2012	us-gaap	Increase Decrease In Regulatory Assets And Liabilities IncreaseDecreaseInRegulatoryAssetsAndLiabilities	259853000	USD
CY2010	us-gaap	Increase Decrease In Regulatory Assets And Liabilities IncreaseDecreaseInRegulatoryAssetsAndLiabilities	-26289000	USD
CY2012	us-gaap	Increase Decrease In Fair Value Of Unhedged Derivative Instruments IncreaseDecreaseInFairValueOfUnhedgedDerivativeInstruments	10455000	USD
CY2010	us-gaap	Increase Decrease In Fair Value Of Unhedged Derivative Instruments IncreaseDecreaseInFairValueOfUnhedgedDerivativeInstruments	11812000	USD
CY2010	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-72151000	USD
CY2011	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	5820000	USD
CY2012	us-gaap	Increase Decrease In Receivables IncreaseDecreaseInReceivables	20214000	USD
CY2010	us-gaap	Increase Decrease In Receivables IncreaseDecreaseInReceivables	51285000	USD
CY2010	us-gaap	Net Income Loss NetIncomeLoss	394107000	USD
CY2012	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	7132000	USD
CY2010	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	6158000	USD
CY2012	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	525945000	USD
CY2012	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	277993631	shares
CY2011	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	177804568	shares
CY2010	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	176885387	shares
CY2012	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	277209819	shares
CY2011	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	177410167	shares
CY2010	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	176636086	shares
CY2012	us-gaap	Utilities Operating Expense Maintenance And Operations UtilitiesOperatingExpenseMaintenanceAndOperations	1583070000	USD
CY2010	us-gaap	Utilities Operating Expense Maintenance And Operations UtilitiesOperatingExpenseMaintenanceAndOperations	1001349000	USD
CY2010	nu	Energy Efficiency Programs EnergyEfficiencyPrograms	124023000	USD
CY2012	nu	Energy Efficiency Programs EnergyEfficiencyPrograms	313149000	USD
CY2011	nu	Energy Efficiency Programs EnergyEfficiencyPrograms	131415000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Net Of Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax	1971000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Net Of Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax	-14177000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	217000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	506000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	402000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	-4356000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	-13645000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	-505000	USD
CY2011	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	5820000	USD
CY2012	us-gaap	Provision For Doubtful Accounts ProvisionForDoubtfulAccounts	36275000	USD
CY2010	us-gaap	Provision For Doubtful Accounts ProvisionForDoubtfulAccounts	31352000	USD
CY2010	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	-38126000	USD
CY2012	us-gaap	Increase Decrease In Income Taxes Receivable IncreaseDecreaseInIncomeTaxesReceivable	5450000	USD
CY2010	us-gaap	Increase Decrease In Income Taxes Receivable IncreaseDecreaseInIncomeTaxesReceivable	82103000	USD
CY2012	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-128339000	USD
CY2010	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-44355000	USD
CY2012	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-8532000	USD
CY2010	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-17466000	USD
CY2012	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	1161229000	USD
CY2010	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	1093463000	USD
CY2012	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	1472272000	USD
CY2010	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	954472000	USD
CY2012	us-gaap	Proceeds From Sale And Maturity Of Marketable Securities ProceedsFromSaleAndMaturityOfMarketableSecurities	317294000	USD
CY2010	us-gaap	Proceeds From Sale And Maturity Of Marketable Securities ProceedsFromSaleAndMaturityOfMarketableSecurities	174865000	USD
CY2012	us-gaap	Payments To Acquire Marketable Securities PaymentsToAcquireMarketableSecurities	348629000	USD
CY2012	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-35683000	USD
CY2012	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-1467924000	USD
CY2011	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	196761000	USD
CY2010	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	180542000	USD
CY2010	us-gaap	Payments Of Dividends Preferred Stock And Preference Stock PaymentsOfDividendsPreferredStockAndPreferenceStock	5559000	USD
CY2012	us-gaap	Repayments Of Secured Debt RepaymentsOfSecuredDebt	114433000	USD
CY2012	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	6529000	USD
CY2012	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	345884000	USD
CY2012	us-gaap	Cash And Cash Equivalents Period Increase Decrease CashAndCashEquivalentsPeriodIncreaseDecrease	39189000	USD
CY2012	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	USD
CY2010	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	USD
CY2011	us-gaap	Provision For Doubtful Accounts ProvisionForDoubtfulAccounts	16420000	USD
CY2011	us-gaap	Pension And Other Postretirement Benefit Contributions PensionAndOtherPostretirementBenefitContributions	191101000	USD
CY2011	us-gaap	Pension And Other Postretirement Benefit Expense PensionAndOtherPostretirementBenefitExpense	133000000	USD
CY2011	us-gaap	Increase Decrease In Regulatory Assets And Liabilities IncreaseDecreaseInRegulatoryAssetsAndLiabilities	70863000	USD
CY2011	us-gaap	Increase Decrease In Fair Value Of Unhedged Derivative Instruments IncreaseDecreaseInFairValueOfUnhedgedDerivativeInstruments	35441000	USD
CY2011	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-29751000	USD
CY2011	us-gaap	Increase Decrease In Receivables IncreaseDecreaseInReceivables	-17570000	USD
CY2011	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	11033000	USD
CY2011	us-gaap	Increase Decrease In Income Taxes Receivable IncreaseDecreaseInIncomeTaxesReceivable	-49642000	USD
CY2011	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	18916000	USD
CY2011	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-12569000	USD
CY2011	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	970386000	USD
CY2011	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	1076730000	USD
CY2011	us-gaap	Proceeds From Sale And Maturity Of Marketable Securities ProceedsFromSaleAndMaturityOfMarketableSecurities	149441000	USD
CY2011	us-gaap	Payments To Acquire Marketable Securities PaymentsToAcquireMarketableSecurities	151972000	USD
CY2011	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	46841000	USD
CY2011	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-13833000	USD
CY2011	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-1018587000	USD
CY2010	us-gaap	Proceeds From Repayments Of Short Term Debt ProceedsFromRepaymentsOfShortTermDebt	166687000	USD
CY2010	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	145000000	USD
CY2011	us-gaap	Repayments Of Secured Debt RepaymentsOfSecuredDebt	69312000	USD
CY2011	us-gaap	Repayments Of Long Term Debt RepaymentsOfLongTermDebt	369586000	USD
CY2011	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-7123000	USD
CY2011	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	31365000	USD
CY2011	us-gaap	Cash And Cash Equivalents Period Increase Decrease CashAndCashEquivalentsPeriodIncreaseDecrease	-16836000	USD
CY2010	us-gaap	Payments To Acquire Marketable Securities PaymentsToAcquireMarketableSecurities	177204000	USD
CY2010	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	1157000	USD
CY2010	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-957968000	USD
CY2009Q4	us-gaap	Cash Cash	26952000	USD
CY2010Q4	us-gaap	Cash Cash	23395000	USD
CY2012	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	375047000	USD
CY2012	us-gaap	Payments Of Dividends Preferred Stock And Preference Stock PaymentsOfDividendsPreferredStockAndPreferenceStock	7029000	USD
CY2012	us-gaap	Proceeds From Repayments Of Short Term Debt ProceedsFromRepaymentsOfShortTermDebt	825000000	USD
CY2012	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	850000000	USD
CY2011	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	194555000	USD
CY2011	us-gaap	Payments Of Dividends Preferred Stock And Preference Stock PaymentsOfDividendsPreferredStockAndPreferenceStock	5559000	USD
CY2011	us-gaap	Proceeds From Repayments Of Short Term Debt ProceedsFromRepaymentsOfShortTermDebt	50000000	USD
CY2011	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	627500000	USD
CY2010	us-gaap	Repayments Of Long Term Debt RepaymentsOfLongTermDebt	4286000	USD
CY2010	us-gaap	Repayments Of Secured Debt RepaymentsOfSecuredDebt	260864000	USD
CY2010	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	512000	USD
CY2010	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-139052000	USD
CY2010	us-gaap	Cash And Cash Equivalents Period Increase Decrease CashAndCashEquivalentsPeriodIncreaseDecrease	-3557000	USD
CY2012	us-gaap	Pension And Other Postretirement Benefit Contributions PensionAndOtherPostretirementBenefitContributions	295028000	USD
CY2010	us-gaap	Pension And Other Postretirement Benefit Contributions PensionAndOtherPostretirementBenefitContributions	90633000	USD
CY2012Q4	us-gaap	Restricted Cash And Cash Equivalents RestrictedCashAndCashEquivalents	3300000	USD
CY2011Q4	us-gaap	Restricted Cash And Cash Equivalents RestrictedCashAndCashEquivalents	17900000	USD
CY2012Q4	nu	Cash Collateral Deposited CashCollateralDeposited	14600000	USD
CY2011Q4	nu	Cash Collateral Deposited CashCollateralDeposited	10900000	USD
CY2012Q4	nu	Rate Reduction Bond Deposits RateReductionBondDeposits	69400000	USD
CY2012	nu	Description Of Consolidation Of Yankee Companies DescriptionOfConsolidationOfYankeeCompanies	On April 10, 2012, upon consummation of the merger with NSTAR, NSTAR Electric's ownership in CYAPC and YAEC combined with CL&P's, PSNH's and WMECO's respective ownership interests in CYAPC and YAEC totaled greater than 50 percent, requiring NU to consolidate CYAPC and YAEC from April 10, 2012 and forward. The investment in CYAPC and YAEC had previously been accounted for under the equity method by NU. The consolidation of CYAPC and YAEC results in NU recording nuclear decommissioning trust marketable securities of $340.4 million, regulatory assets of $214 million, long-term debt associated with the long-term spent nuclear fuel disposal liabilities of $179.3 million, net accumulated deferred income tax liability of $56.4 million and asset retirement obligations related to decommissioning activity of $311.4 million as of December 31, 2012. At the NU consolidated level, intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation. For CL&P, NSTAR Electric, PSNH and WMECO, the investment in CYAPC and YAEC continue to be accounted for under the equity method. See Note 1J, "Summary of Significant Accounting Policies – Equity Method Investments," for further information.
CY2012	us-gaap	Immaterial Error Correction ImmaterialErrorCorrection	The NSTAR Electric consolidated statements of cash flows were revised to correct an error in the presentation of cash deposits related to the RRBs.The impact of this revision was an increase in investing cash inflows from Other Investing Activities in an amount of $1.7 million and $24.1 million and a corresponding increase to financing cash outflows from Retirements ofRate Reduction Bonds for the years ended December 31, 2011 and 2010, respectively. These revisions had no impact on NSTAR Electric’s results of operations or cash balance and are not deemed material, individually orin the aggregate, to the previously issued consolidated financial statements.
CY2011	nu	Reclassification Of Transmission Expense ReclassificationOfTransmissionExpense	77200000	USD
CY2010	nu	Reclassification Of Transmission Expense ReclassificationOfTransmissionExpense	48900000	USD
CY2011	nu	Reclassification Of Energy Expense ReclassificationOfEnergyExpense	131400000	USD
CY2010	nu	Reclassification Of Energy Expense ReclassificationOfEnergyExpense	124000000	USD
CY2011Q4	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	115700000	USD
CY2012Q4	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	165500000	USD
CY2011Q4	nu	Rate Reduction Bond Deposits RateReductionBondDeposits	29500000	USD
CY2012	nu	Fuel Power Costs FuelPowerCosts	346800000	USD
CY2011	nu	Fuel Power Costs FuelPowerCosts	307900000	USD
CY2010	nu	Fuel Power Costs FuelPowerCosts	391600000	USD
CY2012	us-gaap	Public Utilities Allowance For Funds Used During Construction Capitalized Interest PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest	-5300000	USD
CY2012	us-gaap	Public Utilities Allowance For Funds Used During Construction Capitalized Cost Of Equity PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity	6800000	USD
CY2012	nu	Public Utilities Allowance For Funds Used During Construction Capitalized Total PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedTotal	12100000	USD
CY2011	us-gaap	Public Utilities Allowance For Funds Used During Construction Capitalized Interest PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest	-11800000	USD
CY2011	us-gaap	Public Utilities Allowance For Funds Used During Construction Capitalized Cost Of Equity PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity	22500000	USD
CY2011	nu	Public Utilities Allowance For Funds Used During Construction Capitalized Total PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedTotal	34300000	USD
CY2010	us-gaap	Public Utilities Allowance For Funds Used During Construction Capitalized Interest PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest	-10200000	USD
CY2010	us-gaap	Public Utilities Allowance For Funds Used During Construction Capitalized Cost Of Equity PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity	16700000	USD
CY2010	nu	Public Utilities Allowance For Funds Used During Construction Capitalized Total PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedTotal	26900000	USD
CY2012	us-gaap	Public Utilities Allowance For Funds Used During Construction Rate PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate	0.037	pure
CY2011	us-gaap	Public Utilities Allowance For Funds Used During Construction Rate PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate	0.073	pure
CY2010	us-gaap	Public Utilities Allowance For Funds Used During Construction Rate PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate	0.071	pure
CY2012	us-gaap	Excise And Sales Taxes ExciseAndSalesTaxes	135000000	USD
CY2011	us-gaap	Excise And Sales Taxes ExciseAndSalesTaxes	137800000	USD
CY2010	us-gaap	Excise And Sales Taxes ExciseAndSalesTaxes	143700000	USD
CY2012Q4	us-gaap	Business Acquisition Purchase Price Allocation Other Noncurrent Liabilities BusinessAcquisitionPurchasePriceAllocationOtherNoncurrentLiabilities	2700000000	USD
CY2012Q4	us-gaap	Business Acquisition Purchase Price Allocation Noncurrent Liabilities Long Term Debt BusinessAcquisitionPurchasePriceAllocationNoncurrentLiabilitiesLongTermDebt	2100000000	USD
CY2012Q4	us-gaap	Business Acquisition Cost Of Acquired Entity Purchase Price BusinessAcquisitionCostOfAcquiredEntityPurchasePrice	5000000000	USD
CY2012	nu	Generation Average Depreciable Lives GenerationAverageDepreciableLives	32.7	pure
CY2012	nu	Other Average Depreciable Lives OtherAverageDepreciableLives	16.7	pure
CY2012	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	160600000	USD
CY2011	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	168500000	USD
CY2010	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	127900000	USD
CY2012	us-gaap	Interest Paid Net InterestPaidNet	356500000	USD
CY2011	us-gaap	Interest Paid Net InterestPaidNet	256300000	USD
CY2010	us-gaap	Interest Paid Net InterestPaidNet	258300000	USD
CY2010	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	84500000	USD
CY2011	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	-76600000	USD
CY2012	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	-12800000	USD
CY2012Q2	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	314000000	shares
CY2012Q2	nu	Number Of Nstar Shares Outstanding NumberOfNstarSharesOutstanding	103696000	shares
CY2012Q2	nu	Exchange Ratio ExchangeRatio	1.312	pure
CY2012Q2	us-gaap	Business Acquisition Equity Interests Issued Or Issuable Number Of Shares Issued BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued	136049000	shares
CY2012Q2	us-gaap	Business Acquisition Share Price BusinessAcquisitionSharePrice	36.79
CY2012Q2	us-gaap	Business Acquisition Cost Of Acquired Entity Equity Interests Issued And Issuable BusinessAcquisitionCostOfAcquiredEntityEquityInterestsIssuedAndIssuable	5005000000	USD
CY2012Q2	us-gaap	Business Acquisition Cost Of Acquired Entity Other Noncash Consideration BusinessAcquisitionCostOfAcquiredEntityOtherNoncashConsideration	33000000	USD
CY2012Q2	us-gaap	Business Acquisition Cost Of Acquired Entity Purchase Price BusinessAcquisitionCostOfAcquiredEntityPurchasePrice	5038000000	USD
CY2012Q2	us-gaap	Business Acquisition Percentage Of Voting Interests Acquired BusinessAcquisitionPercentageOfVotingInterestsAcquired	1.00	pure
CY2012Q2	nu	Business Acquisition Number Of Shares Awarded Vested Immediately Prior To Merger BusinessAcquisitionNumberOfSharesAwardedVestedImmediatelyPriorToMerger	109000	shares
CY2012Q4	us-gaap	Business Acquisition Purchase Price Allocation Current Assets BusinessAcquisitionPurchasePriceAllocationCurrentAssets	700000000	USD
CY2012Q4	us-gaap	Business Acquisition Purchase Price Allocation Property Plant And Equipment BusinessAcquisitionPurchasePriceAllocationPropertyPlantAndEquipment	5100000000	USD
CY2012Q4	us-gaap	Business Acquisition Purchase Price Allocation Goodwill Amount BusinessAcquisitionPurchasePriceAllocationGoodwillAmount	3200000000	USD
CY2012Q4	us-gaap	Business Acquisition Purchase Price Allocation Other Noncurrent Assets BusinessAcquisitionPurchasePriceAllocationOtherNoncurrentAssets	2100000000	USD
CY2012Q4	us-gaap	Business Acquisition Purchase Price Allocation Current Liabilities BusinessAcquisitionPurchasePriceAllocationCurrentLiabilities	1300000000	USD
CY2012	us-gaap	Business Acquisitions Pro Forma Revenue BusinessAcquisitionsProFormaRevenue	7004000000	USD
CY2011	us-gaap	Business Acquisitions Pro Forma Revenue BusinessAcquisitionsProFormaRevenue	7361000000	USD
CY2012	us-gaap	Business Acquisitions Pro Forma Net Income Loss BusinessAcquisitionsProFormaNetIncomeLoss	630000000	USD
CY2011	us-gaap	Business Acquisitions Pro Forma Net Income Loss BusinessAcquisitionsProFormaNetIncomeLoss	689000000	USD
CY2012	us-gaap	Business Acquisition Pro Forma Earnings Per Share Basic BusinessAcquisitionProFormaEarningsPerShareBasic	2.00
CY2012	us-gaap	Business Acquisition Pro Forma Earnings Per Share Diluted BusinessAcquisitionProFormaEarningsPerShareDiluted	1.99
CY2011	us-gaap	Business Acquisition Pro Forma Earnings Per Share Diluted BusinessAcquisitionProFormaEarningsPerShareDiluted	2.19
CY2011	us-gaap	Business Acquisition Pro Forma Earnings Per Share Basic BusinessAcquisitionProFormaEarningsPerShareBasic	2.20
CY2012	nu	Business Combination After Tax Transaction Costs BusinessCombinationAfterTaxTransactionCosts	32000000	USD
CY2011	nu	Business Combination After Tax Transaction Costs BusinessCombinationAfterTaxTransactionCosts	19000000	USD
CY2012	nu	After Tax Settlement Costs Incurred AfterTaxSettlementCostsIncurred	60000000	USD
CY2011	nu	After Tax Settlement Costs Incurred AfterTaxSettlementCostsIncurred	0	USD
CY2012	nu	After Tax Costs Transaction And Other Non Recurring Costs Excluded From Pro Forma Net Income AfterTaxCostsTransactionAndOtherNonRecurringCostsExcludedFromProFormaNetIncome	92000000	USD
CY2011	nu	After Tax Costs Transaction And Other Non Recurring Costs Excluded From Pro Forma Net Income AfterTaxCostsTransactionAndOtherNonRecurringCostsExcludedFromProFormaNetIncome	19000000	USD
CY2011Q4	us-gaap	Regulatory Assets RegulatoryAssets	3522800000	USD
CY2012	nu	Description Of Accumulated Equity Return DescriptionOfAccumulatedEquityReturn	Equity Return on Regulatory Assets: For rate-making purposes, the Regulated companies recover the carrying cost, including an allowed equity return, on certain regulatory assets. This equity return, which is not recorded on the accompanying consolidated balance sheets, totaled $2.5 million and $3.5 million for CL&P and $21.8 million and $7.6 million for PSNH as of December 31, 2012 and 2011, respectively. These carrying costs will be recovered in future rates.
CY2012	us-gaap	Description Of Regulatory Assets And Liabilities DescriptionOfRegulatoryAssetsAndLiabilities	Regulatory Assets - The following provides further information about regulatory assets: Benefit Costs: NU's Pension, SERP and PBOP Plans are accounted for in accordance with accounting guidance on defined benefit pension and other postretirement plans. Under this accounting guidance, the funded status of pension and other postretirement plans is recorded with an offset to Accumulated Other Comprehensive Income/(Loss) and is remeasured annually. However, because the Regulated companies recover these costs from customers through rates, regulatory assets are recorded as an offset for the liability that is recognized for the funded status of the pension and postretirement plans. Regulatory accounting was also applied to the portions of the NUSCO and NSTAR Electric & Gas costs that support the Regulated companies, as these amounts are also recoverable. CL&P and PSNH do not collect carrying charges on these deferred benefit costs regulatory assets. WMECO's deferred benefit costs regulatory assets are earning a return at the same rate as the assets included in rate base. NSTAR Electric does not earn a return on the regulatory assets recorded to offset the funded status. NSTAR Electric and WMECO each recover their qualified pension and postretirement expenses through rate reconciling mechanisms that fully track the change in net pension and postretirement expenses each year. CL&P and PSNH will recover benefit costs through rates as allowed by their applicable regulatory commissions. NSTAR Electric earns a carrying charge on the excess cumulative benefit plan trust fund contributions it has made over what it has cumulatively recognized as net periodic benefit expense, net of deferred income taxes. As of December 31, 2012 and 2011, these balances were $366.8 million and $428 million of the benefit costs regulatory asset, respectively. Regulatory Assets Offsetting Derivative Liabilities: The regulatory assets offsetting derivative liabilities relate to the fair value of contracts used to purchase power and other related contracts that will be collected from customers in the future. See Note 5, "Derivative Instruments," to the consolidated financial statements for further information.  These assets are excluded from rate base and are being recovered as the actual settlement occurs over the duration of the contracts. Goodwill: Goodwill that originated from the merger that created NSTAR in 1999 is recoverable in rates over the remaining 27 year amortization period, without a carrying charge. Storm Restoration Costs: The storm restoration cost deferrals relate to costs incurred at CL&P, NSTAR Electric, PSNH and WMECO for restorations that the Company expects to collect from customers. A storm must meet certain criteria to be declared a major storm with the criteria specific to each state jurisdiction and utility company. Once a storm is declared major, all qualifying expenses incurred during storm restoration efforts, if deemed prudent, are deferred and recovered from customers in future periods. In Connecticut, qualifying storm restoration costs must exceed $5 million for a storm to be declared a major storm. In Massachusetts, qualifying storm restoration costs must exceed $1 million for NSTAR Electric and $300,000 for WMECO and an emergency response plan must be initiated for a storm to be declared a major storm. In New Hampshire, (1) at least 10 percent of customers must be without power with at least 200 concurrent locations requiring repairs (trouble spots), or (2) at least 300 concurrent trouble spots must be reported for a storm to be declared a major storm. In 2011, Tropical Storm Irene and the October snowstorm each caused extensive damage to NU’s distribution system. As of December 31, 2012 and 2011, CL&P had recorded total deferred storm restoration costs relating to Tropical Storm Irene and the October 2011 snowstorm as a regulatory asset of $281.6 million and $263.3 million, respectively. The CL&P storm restoration cost regulatory asset balance includes a reserve of $40 million recorded in connection with the Connecticut settlement agreement. See Note 2, "Merger of NU and NSTAR," for further information. As of December 31, 2012 and 2011, NSTAR Electric had recorded total deferred storm restoration costs for these 2011 storms of $35.8 million and $35.8 million, respectively, and WMECO had recorded $26.5 million and $26.7 million, respectively, as regulatory assets. PSNH recorded $22.3 million and $21.7 million for these 2011 storms in Other Long-Term Assets, as of December 31, 2012 and 2011, respectively, as previously described. On August 1, 2012, PURA issued a final decision in the investigation of CL&P’s performance related to both Tropical Storm Irene and the October 2011 snowstorm. The decision concluded that CL&P was deficient and inadequate in its preparation, response, and communication to both storms, and identified certain penalties that could be imposed on CL&P during its next rate case, including a reduction in allowed regulatory ROE and the disallowance of certain deferred storm restoration costs. However, PURA will consider and weigh the extent to which CL&P has taken steps in its restructuring of storm management and the establishment of new practices for execution in future storm response in determining any potential penalties. CL&P believes such steps to improve current storm preparation and response practices have been successfully executed in recent storms. At this time, management cannot estimate the impact on CL&P’s financial position, results of operations or cash flows. CL&P continues to believe that its response to these 2011 storms was prudent, was consistent with industry standards, and that it is probable that it will be able to recover its deferred costs. See Note 12E, "Commitments and Contingencies – DPU Penalties for 2011 Storm Responses," for a discussion of NSTAR Electric and WMECO's 2011 storm response. On October 29, 2012, Hurricane Sandy caused extensive damage to NU’s electric distribution system across all three states. The cost of restoration that was deferred for future recovery from customers and recorded as a regulatory asset as of December 31, 2012 for CL&P, NSTAR Electric, and WMECO totaled $159.9 million, $27.8 million and $4.2 million, respectively. PSNH recorded $12.1 million in Other-Long Term Assets, as previously described. Management believes its response to the storm damage was prudent and therefore believes it is probable that CL&P, NSTAR Electric, PSNH and WMECO will be allowed to recover these deferred storm restoration costs. Accordingly, the storm did not have a material impact to the results of operations of CL&P, NSTAR Electric, PSNH or WMECO. Each operating company will seek recovery of these deferred storm restoration costs through its applicable regulatory recovery process. The PSNH storm restoration costs deferral as of December 31, 2012 and 2011 related to costs incurred for a major storm in December 2008 and the February 2010 wind storm, both of which were approved for recovery and are included in rate base. Income Taxes, Net: The tax effect of temporary book-tax differences (differences between the periods in which transactions affect income in the financial statements and the periods in which they affect the determination of taxable income, including those differences relating to uncertain tax positions) is accounted for in accordance with the rate-making treatment of the applicable regulatory commissions and accounting guidance for income taxes. Differences in income taxes between the accounting guidance and the rate-making treatment of the applicable regulatory commissions are recorded as regulatory assets. As these assets are offset by deferred income tax liabilities, no carrying charge is collected. For further information regarding income taxes, see Note 11, "Income Taxes," to the consolidated financial statements. Securitized Assets: In March 2005, NSTAR Electric issued $674.5 million RRBs and used the majority of the proceeds from that issuance to effect purchase power contract buyouts. The collateralized amounts reflected as securitized regulatory assets for NSTAR Electric as of December 31, 2012 and 2011 were $14.1 million and $98.4 million, respectively. In April 2001, PSNH issued $525 million RRBs and used the majority of the proceeds from that issuance to buydown its power contracts with an affiliate, North Atlantic Energy Corporation. In May 2001, WMECO issued $155 million RRBs and used the majority of the proceeds from that issuance to buyout an IPP contract. These assets are not earning an equity return and are being recovered over the amortization period of their associated RRBs. NSTAR Electric RRBs are scheduled to fully amortize by March 15, 2013, PSNH RRBs are scheduled to fully amortize by May 1, 2013, and WMECO RRBs are scheduled to fully amortize by June 1, 2013. NSTAR Electric's remaining balance primarily includes other costs related to purchase power contract divestitures and certain costs related to NSTAR Electric’s former generation business that are recovered with a return through the transition charge and amounted to $186.1 million and $259.8 million as of December 31, 2012 and 2011, respectively. These cost recoveries primarily occur through September 2016 for NSTAR Electric and are subject to adjustment by the DPU. Contractual Obligations: Under the terms of contracts with CYAPC, YAEC and MYAPC, CL&P, NSTAR Electric, PSNH and WMECO are responsible for their proportionate share of the remaining costs of the nuclear facilities, including decommissioning. A portion of these amounts was recorded as contractual obligations regulatory assets. These obligations for CL&P are earning a return and are being recovered through the CTA. Amounts for NSTAR Electric are being recovered without a return through the transition charge and are anticipated to be recovered by 2015. Amounts for WMECO are being recovered without a return and are anticipated to be recovered by 2013, the scheduled completion date of stranded cost recovery. Amounts for PSNH were fully recovered by 2006. As a result of the April 10, 2012 merger with NSTAR and consolidation of CYAPC and YAEC, NU's regulatory asset balance also includes the regulatory assets of CYAPC and YAEC, which amounted to $214 million as of December 31, 2012. At the NU consolidated level, intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies are eliminated in consolidation. Power Contracts Buy Out Agreements: NSTAR Electric's balance represents the recorded contract termination liability related to certain purchase power contract buy out agreements that NSTAR Electric executed in 2004 and their future recovery through NSTAR Electric’s transition charge. NSTAR Electric does not earn a return on this regulatory asset. The contracts’ termination payments will occur over time and will be collected from customers through NSTAR Electric’s transition charge over the same time period. The cost recovery period of these terminated contracts is through September 2016. PSNH's balance represents payments associated with the termination of various power purchase contracts that were recorded as regulatory assets and are amortized over the remaining life of the contracts. Regulatory Tracker Deferrals: Regulatory tracker deferrals are approved rate mechanisms that allow utilities to recover costs in specific business segments through reconcilable tracking mechanisms that are reviewed at least annually by the applicable regulatory commission. The reconciliation process produces deferrals for future recovery or refund, which can be either under or over-collections to be included in future customer rates each year. Regulatory tracker deferrals are recorded as regulatory assets if costs are in excess of collections from customers and are recorded as regulatory liabilities if collections from customers are in excess of costs. All material regulatory tracker deferrals that are in a regulatory asset position are earning a return. The following regulatory reconciliation mechanisms were recorded as either regulatory assets or liabilities as of December 31, 2012 and 2011: CL&P: The PURA has established several reconciliation mechanisms, which allow CL&P to recover costs associated with the procurement of energy for SS and LRS, congestion and other costs associated with power market rules approved by the FERC or as approved by the PURA, C&LM programs, the retail transmission of energy, certain regulatory and energy public policy costs, such as hardship protection costs and transition period property taxes, and stranded costs, such as the amortization of regulatory assets and IPP over market costs. As part of the CTA mechanism reconciliation process, CL&P had also established an obligation to refund the variable incentive portion of its transition service procurement fee, which totaled $26.3 million as of December 31, 2011 and was recorded as a regulatory liability. During 2012, PURA issued a decision approving a joint settlement agreement submitted October 2, 2012, by CL&P, UI, and the Connecticut Consumer Counsel, in resolution of all issues associated with the procurement incentive for 2004, 2005 and 2006. Under the joint settlement agreement, CL&P refunded to customers $5.7 million of funds collected and associated interest. CL&P will be allowed to retain approximately $11.5 million of procurement incentive along with the remaining accrued interest that it was not required to refund to customers. NSTAR Electric and WMECO: Each company recovers certain of its costs on a fully reconciling basis through DPU-approved cost recovery mechanisms. These rate mechanisms recover costs associated with the procurement of energy for basic service, the retail transmission of energy, costs associated with electric industry restructuring, pension and postretirement benefits, and energy efficiency programs. Costs associated with industry restructuring include RRB debt service, nuclear decommissioning costs and above-market IPP costs. In addition, WMECO recovers costs associated with its investments in renewable energy, such as solar projects and credits given to customers who generate renewable energy. In the January 31, 2011 rate case, WMECO received approval for a revenue decoupling reconciliation mechanism, which provides assurance that WMECO will recover a DPU pre-established level of baseline distribution delivery service revenue to manage all other distribution operating expenses and earn a level of return on its capital investment. PSNH: The NHPUC permits PSNH to recover the costs of providing generation, restructuring costs as a result of deregulation, the retail transmission of energy, and the cost of C&LM programs through various reconciliation mechanisms. Asset Retirement Obligations: The costs associated with the depreciation of the Regulated companies' ARO assets and accretion of the ARO liabilities are recorded as regulatory assets in accordance with regulatory accounting guidance. For CL&P, NSTAR Electric and WMECO, ARO assets, regulatory assets and liabilities offset and are excluded from rate base. PSNH's ARO assets, regulatory assets and liabilities are included in rate base. These costs are being recovered over the life of the underlying property, plant and equipment. Other Regulatory Assets: Other Regulatory Assets primarily include environmental remediation costs, losses associated with the reacquisition or redemption of long-term debt, and costs related to previously recognized lost tax benefits as a result of a provision in the 2010 Healthcare Act that eliminated the tax deductibility of actuarially equivalent Medicare Part D benefits for retirees, partially offset by purchase price adjustments recorded in connection with the merger with NSTAR reflected in regulatory assets. Cost of Removal: NU's Regulated companies currently recover amounts in rates for future costs of removal of plant assets over the lives of the assets. The estimated cost to remove utility assets from service is recognized as a component of depreciation expense and the cumulative amounts collected from customers but not yet expended is recognized as a regulatory liability. Expended costs that exceed amounts collected from customers are recognized as regulatory assets, as they are probable of recovery in future rates. AFUDC Transmission Incentive: AFUDC was recorded on 100 percent of CL&P and WMECO's CWIP for their NEEWS projects through May 31, 2011, all of which was reserved as a regulatory liability to reflect rate base recovery for 100 percent of the CWIP as a result of FERC-approved transmission incentives. Effective June 1, 2011, FERC approved changes to the ISO-NE Tariff in order to include 100 percent of the NEEWS CWIP in regional rate base. As a result, CL&P and WMECO no longer record AFUDC on NEEWS CWIP. NSTAR Electric recorded AFUDC on reliability-related projects over $5 million through December 31, 2012, 50 percent of which was reserved as a regulatory liability to reflect rate base recovery for 50 percent of the CWIP as a result of FERC-approved transmission incentives. Spent Nuclear Fuel Costs and Contractual Obligations: CL&P and WMECO currently recover amounts in rates for costs of disposal of spent nuclear fuel and high-level radioactive waste for the period prior to the sale of their ownership shares in the Millstone nuclear power stations. Collections in excess of these costs are recorded as regulatory liabilities. CL&P has also established a regulatory liability for the overrecovery of its proportionate share of the remaining costs, including decommissioning, of the MYAPC nuclear facility. Wholesale Transmission Overcollections: CL&P, NSTAR Electric, PSNH and WMECO's transmission rates recover total transmission revenue requirements, recovering all regional and local revenue requirements for providing transmission service. These rates provide for annual reconciliations to actual costs and the difference between billed and actual costs is deferred. Regulatory liabilities are recorded for collections in excess of costs. Regulatory assets are recorded for costs in excess of collections, as they are probable of recovery in future rates. Other Regulatory Liabilities: Other Regulatory Liabilities primarily includes amounts that are subject to various rate reconciling mechanisms that, as of each period end date, would result in refunds to customers.
CY2012	nu	Business Acquisition Description Of Settlement Agreements BusinessAcquisitionDescriptionOfSettlementAgreements	Regulatory Approvals: On February 15, 2012, NU and NSTAR reached comprehensive settlement agreements with the Massachusetts Attorney General and the DOER related to the merger. The Attorney General settlement agreement covered a variety of rate-making and rate design issues, including a base distribution rate freeze through 2015 for NSTAR Electric, NSTAR Gas and WMECO and $15 million, $3 million and $3 million in the form of rate credits to their respective customers. The settlement agreement reached with the DOER covered the same rate-making and rate design issues as the Attorney General's settlement agreement, as well as a variety of matters impacting the advancement of Massachusetts clean energy policy established by the Green Communities Act and Global Warming Solutions Act. On April 4, 2012, the DPU approved the settlement agreements and the merger of NU and NSTAR. On March 13, 2012, NU and NSTAR reached a comprehensive settlement agreement with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel related to the merger. The settlement agreement covered a variety of matters, including a $25 million rate credit to CL&P customers, a CL&P base distribution rate freeze until December 1, 2014, and the establishment of a $15 million fund for energy efficiency and other initiatives to be disbursed at the direction of the DEEP. In the agreement, CL&P agreed to forego rate recovery of $40 million of the deferred storm restoration costs associated with restoration activities following Tropical Storm Irene and the October 2011 snowstorm. On April 2, 2012, the PURA approved the settlement agreement and the merger of NU and NSTAR.
CY2012	nu	Customer Rate Credits For Business Combination CustomerRateCreditsForBusinessCombination	46000000	USD
CY2012	nu	Storm Cost Deferral Reduction StormCostDeferralReduction	40000000	USD
CY2012	nu	Establishment Of Energy Efficiency Fund EstablishmentOfEnergyEfficiencyFund	15000000	USD
CY2012	nu	Total Pre Tax Settlement Agreement Impacts TotalPreTaxSettlementAgreementImpacts	101000000	USD
CY2012	us-gaap	Goodwill Acquired During Period GoodwillAcquiredDuringPeriod	3200000000	USD
CY2012Q4	us-gaap	Regulatory Assets RegulatoryAssets	5837400000	USD
CY2012	us-gaap	Description Of Regulatory Costs Not Yet Approved DescriptionOfRegulatoryCostsNotYetApproved	Regulatory Costs Not Yet Approved: Additionally, the Regulated companies had $69.9 million ($3.9 million for CL&P, $25.4 million for NSTAR Electric, $35.7 million for PSNH, and $1.4 million for WMECO) and $32.4 million ($5 million for CL&P, $22.4 million for PSNH, and $1.6 million for WMECO) of regulatory costs as of December 31, 2012 and 2011, respectively, which were included in Other Long-Term Assets on the accompanying consolidated balance sheets. For comparative purposes, NSTAR Electric had $9.5 million of such regulatory costs as of December 31, 2011. These amounts represent incurred costs that have not yet been approved for recovery by the applicable regulatory agency. Management believes it is probable that recovery of these costs will ultimately be approved. For PSNH, of the total December 31, 2012 regulatory costs not yet approved, $12.1 million related to costs incurred for the 2012 Hurricane Sandy storm and $22.3 million related to costs incurred for the 2011 Tropical Storm Irene and the October snowstorm restorations that met the NHPUC criteria for cost deferral. As of December 31, 2011, the storm restoration costs incurred for the 2011 Tropical Storm Irene and the October snowstorm restorations totaled $21.7 million. Refer to the "Storm Restoration Costs" section below for further discussion. The NSTAR Electric balance as of December 31, 2012 and 2011 related to costs deferred in connection with the basic service bad debt adder. See Note 12H, "Commitments and Contingencies – Basic Service Bad Debt Adder," for further information.
CY2012Q4	us-gaap	Regulatory Liabilities RegulatoryLiabilities	674300000	USD
CY2011Q4	us-gaap	Regulatory Liabilities RegulatoryLiabilities	433900000	USD
CY2012	us-gaap	Property Plant And Equipment Depreciation Methods PropertyPlantAndEquipmentDepreciationMethods	Utility property, plant and equipment is recorded at original cost. Original cost includes materials, labor, construction overhead and AFUDC for regulated property. The cost of repairs and maintenance, including planned major maintenance activities, is charged to Operating Expenses as incurred. Depreciation of utility assets is calculated on a straight-line basis using composite rates based on the estimated remaining useful lives of the various classes of property (estimated useful life for PSNH distribution). The composite rates are subject to approval by the appropriate state regulatory agency. The composite rates include a cost of removal component, which is collected from customers during the life of the property and is recognized as a regulatory liability. Depreciation rates are applied to property from the time it is placed in service. Upon retirement from service, the cost of the utility asset is charged to the accumulated provision for depreciation. The actual incurred removal costs are applied against the related regulatory liability.
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Distribution PublicUtilitiesPropertyPlantAndEquipmentDistribution	11438200000	USD
CY2012Q4	nu	Public Utilities Property Plant And Equipment Distribution Natural Gas PublicUtilitiesPropertyPlantAndEquipmentDistributionNaturalGas	2274200000	USD
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Transmission PublicUtilitiesPropertyPlantAndEquipmentTransmission	5541100000	USD
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Generation Or Processing PublicUtilitiesPropertyPlantAndEquipmentGenerationOrProcessing	1146600000	USD
CY2012Q4	nu	Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total PublicUtilitiesPropertyPlantAndEquipmentElectricAndNaturalGasUtilityTotal	20400100000	USD
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Other Property Plant And Equipment PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment	429300000	USD
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Plant In Service PublicUtilitiesPropertyPlantAndEquipmentPlantInService	20829400000	USD
CY2012Q4	nu	Electric And Natural Gas Utility Accumulated Depreciation ElectricAndNaturalGasUtilityAccumulatedDepreciation	5065100000	USD
CY2012Q4	nu	Other Accumulated Depreciation OtherAccumulatedDepreciation	171500000	USD
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Accumulated Depreciation PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation	5236600000	USD
CY2012Q4	nu	Property Plant And Equipment Net Excluding Construction Work In Progress PropertyPlantAndEquipmentNetExcludingConstructionWorkInProgress	15592800000	USD
CY2012Q4	us-gaap	Public Utilities Property Plant And Equipment Construction Work In Progress PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress	1012200000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Distribution PublicUtilitiesPropertyPlantAndEquipmentDistribution	6540400000	USD
CY2011Q4	nu	Public Utilities Property Plant And Equipment Distribution Natural Gas PublicUtilitiesPropertyPlantAndEquipmentDistributionNaturalGas	1247600000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Transmission PublicUtilitiesPropertyPlantAndEquipmentTransmission	3541900000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Generation Or Processing PublicUtilitiesPropertyPlantAndEquipmentGenerationOrProcessing	1096000000	USD
CY2011Q4	nu	Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total PublicUtilitiesPropertyPlantAndEquipmentElectricAndNaturalGasUtilityTotal	12425900000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Other Property Plant And Equipment PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment	305100000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Plant In Service PublicUtilitiesPropertyPlantAndEquipmentPlantInService	12731000000	USD
CY2011Q4	nu	Electric And Natural Gas Utility Accumulated Depreciation ElectricAndNaturalGasUtilityAccumulatedDepreciation	3035500000	USD
CY2011Q4	nu	Other Accumulated Depreciation OtherAccumulatedDepreciation	120200000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Accumulated Depreciation PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation	3155700000	USD
CY2011Q4	nu	Property Plant And Equipment Net Excluding Construction Work In Progress PropertyPlantAndEquipmentNetExcludingConstructionWorkInProgress	9575300000	USD
CY2011Q4	us-gaap	Public Utilities Property Plant And Equipment Construction Work In Progress PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress	827800000	USD
CY2012	nu	Public Utilities Property Plant And Equipment Depreciation Rates PublicUtilitiesPropertyPlantAndEquipmentDepreciationRates	0.025	pure
CY2011	nu	Public Utilities Property Plant And Equipment Depreciation Rates PublicUtilitiesPropertyPlantAndEquipmentDepreciationRates	0.026	pure
CY2010	nu	Public Utilities Property Plant And Equipment Depreciation Rates PublicUtilitiesPropertyPlantAndEquipmentDepreciationRates	0.027	pure
CY2012	nu	Distribution Average Depreciable Lives DistributionAverageDepreciableLives	42.1	pure
CY2012	nu	Transmission Average Depreciable Lives TransmissionAverageDepreciableLives	45.3	pure
CY2012Q4	nu	Current Derivative Assets Consolidated CurrentDerivativeAssetsConsolidated	11400000	USD
CY2011Q4	nu	Current Derivative Assets Consolidated CurrentDerivativeAssetsConsolidated	11000000	USD
CY2012Q4	nu	Derivative Assets Subject To Credit Risk With Investment Grade Counterparty DerivativeAssetsSubjectToCreditRiskWithInvestmentGradeCounterparty	96500000	USD
CY2012	us-gaap	Description Of Derivative Activity Volume DescriptionOfDerivativeActivityVolume	As of December 31, 2012 and 2011, NU had approximately 24 thousand MWh and 123 thousand MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with supply contracts.
CY2012	nu	Interest Realized In Earnings On Fair Value Hedge InterestRealizedInEarningsOnFairValueHedge	2500000	USD
CY2011	nu	Interest Realized In Earnings On Fair Value Hedge InterestRealizedInEarningsOnFairValueHedge	10500000	USD
CY2010	nu	Interest Realized In Earnings On Fair Value Hedge InterestRealizedInEarningsOnFairValueHedge	10900000	USD
CY2011Q4	us-gaap	Notional Amount Of Fair Value Hedge Instruments NotionalAmountOfFairValueHedgeInstruments	263000000	USD
CY2011	us-gaap	Description Of Cash Flow Hedge Activity DescriptionOfCashFlowHedgeActivity	In 2011, PSNH and WMECO settled interest rate swaps associated with $280 million and $50 million, respectively, of long-term debt issuances and as a result PSNH and WMECO recorded pre-tax reductions of $18.2 million and $6.9 million, respectively, to AOCI that are being amortized over the remaining lives of the associated debt. In addition, NU, CL&P, PSNH and WMECO continue to amortize interest rate swaps settled in prior years from AOCI into Interest Expense over the remaining life of the associated long-term debt.
CY2012	us-gaap	Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net CashFlowHedgeGainLossReclassifiedToInterestExpenseNet	-3300000	USD
CY2011	us-gaap	Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net CashFlowHedgeGainLossReclassifiedToInterestExpenseNet	-1300000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Before Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesBeforeTax	-25100000	USD
CY2010	us-gaap	Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net CashFlowHedgeGainLossReclassifiedToInterestExpenseNet	-400000	USD
CY2012Q4	us-gaap	Additional Collateral Aggregate Fair Value AdditionalCollateralAggregateFairValue	17400000	USD
CY2012Q4	us-gaap	Derivative Net Liability Position Aggregate Fair Value DerivativeNetLiabilityPositionAggregateFairValue	15300000	USD
CY2011Q4	us-gaap	Derivative Net Liability Position Aggregate Fair Value DerivativeNetLiabilityPositionAggregateFairValue	23500000	USD
CY2011Q4	us-gaap	Collateral Already Posted Aggregate Fair Value CollateralAlreadyPostedAggregateFairValue	4100000	USD
CY2011Q4	us-gaap	Additional Collateral Aggregate Fair Value AdditionalCollateralAggregateFairValue	19900000	USD
CY2010Q4	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliations Recurring Basis Liability Value FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue	840200000	USD
CY2011	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements	38500000	USD
CY2011Q4	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliations Recurring Basis Liability Value FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue	962200000	USD
CY2012Q4	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliations Recurring Basis Liability Value FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue	878600000	USD
CY2012	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	10900000	USD
CY2012	nu	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityGainLossIncludedInRegAssetLiability	-29200000	USD
CY2012	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements	75100000	USD
CY2011	nu	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityGainLossIncludedInRegAssetLiability	-161000000	USD
CY2011	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	500000	USD
CY2012	nu	Derivativeliabilityrecordedatmergerdatelevel3 Derivativeliabilityrecordedatmergerdatelevel3	-5400000	USD
CY2012	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Liability Transfers Out Of Level3 FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3	32200000	USD
CY2011Q4	nu	Fair Value Of Securities Under Fair Value Option FairValueOfSecuritiesUnderFairValueOption	41100000	USD
CY2012	us-gaap	Fair Value Option Changes In Fair Value Gain Loss1 FairValueOptionChangesInFairValueGainLoss1	5900000	USD
CY2011	us-gaap	Fair Value Option Changes In Fair Value Gain Loss1 FairValueOptionChangesInFairValueGainLoss1	-1100000	USD
CY2012Q4	nu	Fair Value Of Securities Under Fair Value Option FairValueOfSecuritiesUnderFairValueOption	47000000	USD
CY2012Q4	us-gaap	Marketable Securities Restricted Noncurrent MarketableSecuritiesRestrictedNoncurrent	340400000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost	66600000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost	57300000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost	51200000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities After Ten Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost	91500000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAmortizedCost	266600000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Fair Value AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue	66600000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	58700000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue	54600000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Maturities After Ten Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue	99900000	USD
CY2012Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	279800000	USD
CY2012Q4	us-gaap	Investments Fair Value Disclosure InvestmentsFairValueDisclosure	492300000	USD
CY2011Q4	us-gaap	Investments Fair Value Disclosure InvestmentsFairValueDisclosure	131300000	USD
CY2011Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	56200000	USD
CY2012	us-gaap	Asset Retirement Obligation Liabilities Settled AssetRetirementObligationLiabilitiesSettled	-7200000	USD
CY2012	us-gaap	Asset Retirement Obligation Accretion Expense AssetRetirementObligationAccretionExpense	20200000	USD
CY2012	us-gaap	Asset Retirement Obligation Revision Of Estimate AssetRetirementObligationRevisionOfEstimate	21400000	USD
CY2012Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	412200000	USD
CY2010Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	53300000	USD
CY2011	us-gaap	Asset Retirement Obligation Liabilities Incurred AssetRetirementObligationLiabilitiesIncurred	2100000	USD
CY2011	us-gaap	Asset Retirement Obligation Accretion Expense AssetRetirementObligationAccretionExpense	3500000	USD
CY2011	us-gaap	Asset Retirement Obligation Revision Of Estimate AssetRetirementObligationRevisionOfEstimate	-1900000	USD
CY2011	us-gaap	Asset Retirement Obligation Liabilities Settled AssetRetirementObligationLiabilitiesSettled	-800000	USD
CY2012Q4	nu	Liability Assumed Upon Consolidation Of Cyapc And Yaec LiabilityAssumedUponConsolidationOfCyapcAndYaec	284200000	USD
CY2012Q4	nu	Liability Assumed Upon Merger With Nstar LiabilityAssumedUponMergerWithNstar	35900000	USD
CY2011Q4	nu	Letters Of Credit Outstanding Under Line Of Credit LettersOfCreditOutstandingUnderLineOfCredit	17900000	USD
CY2012Q4	us-gaap	Commercial Paper CommercialPaper	1150000000	USD
CY2012	nu	Description Of Commercial Paper Program DescriptionOfCommercialPaperProgram	On July 25, 2012, NU, CL&P, NSTAR LLC, NSTAR Gas, PSNH, WMECO, and Yankee Gas jointly entered into a five-year $1.15 billion revolving credit facility. The new facility replaced (1) the NSTAR LLC revolving credit facility of $175 million that served to backstop a commercial paper program utilized by NSTAR LLC and was scheduled to expire on December 31, 2012, (2) the NSTAR Gas revolving credit facility of $75 million that expired on June 8, 2012, and (3) the CL&P, PSNH, WMECO, and Yankee Gas joint three-year $400 million and NU parent three-year $500 million unsecured revolving credit facilities that were scheduled to expire on September 24, 2013. The new facility expires on July 25, 2017. Management expects the new facility to be used primarily to backstop the $1.15 billion commercial paper program at NU, which commenced July 25, 2012. The commercial paper program allows NU parent to issue commercial paper as a form of short-term debt. Under the terms of the agreement, NU parent may provide intercompany loans to its subsidiaries, including CL&P, PSNH and WMECO. On July 25, 2012, NSTAR Electric entered into a five-year $450 million revolving credit facility. This new facility serves to backstop NSTAR Electric’s existing $450 million commercial paper program. The new facility expires on July 25, 2017. This new facility replaced a prior $450 million NSTAR Electric revolving credit facility that was scheduled to expire on December 31, 2012. As of December 31, 2012, NU had $1.15 billion in short-term borrowings outstanding under its commercial paper program. The weighted-average interest rate on these borrowings as of December 31, 2012 was 0.46 percent, which is generally based on money market rates. As of December 31, 2012, there were inter-company loans of $987.5 million from NU to its subsidiaries ($405.1 million for CL&P, $63.3 million for PSNH, and $31.9 million for WMECO). As of December 31, 2012, NSTAR Electric had $276 million in short-term borrowings outstanding under its commercial paper program, leaving $174 million of available borrowing capacity. The weighted-average interest rate on these borrowings as of December 31, 2012 was 0.31 percent, which is generally based on money market rates.
CY2012	nu	Description Of St Debt Reclassified To Lt DescriptionOfStDebtReclassifiedToLt	On January 15, 2013, CL&P issued $400 million of Series A First and Refunding Mortgage Bonds with a coupon rate of 2.5 percent and a maturity date of January 15, 2023. The proceeds, net of issuance costs, were used to pay short-term borrowings outstanding under the CL&P credit agreement and the NU commercial paper program. As a result, as of December 31, 2012, CL&P's credit agreement borrowings of $89 million and intercompany loans related to the commercial paper program of $305.8 million have been classified as Long-Term Debt on the accompanying consolidated balance sheet.
CY2012	nu	Description Of Working Capital DescriptionOfWorkingCapital	NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, pay costs, including storm-related costs, pay dividends and fund other corporate obligations, such as pension contributions. The current growth in NU’s transmission construction expenditures utilizes a significant amount of cash for projects that have a long-term return on investment and recovery period. In addition, NU’s Regulated companies operate in an environment where recovery of its electric and natural gas distribution construction expenditures takes place over an extended period of time. This impacts the timing of the revenue stream designed to fully recover the total investment plus a return on the equity portion of the cost and related financing costs. These factors have resulted in NU’s current liabilities exceeding current assets by approximately $1.4 billion, $268 million, $198 million and $60 million at NU, CL&P, NSTAR Electric and WMECO, respectively, as of December 31, 2012. As of December 31, 2012, approximately $730 million of NU's current liabilities relates to long-term debt that will be paid in the next 12 months, consisting of $550 million for NU parent, $55 million for WMECO, and $125 million for CL&P. Approximately $32 million relates to the amortization of the purchase accounting fair value adjustment that will be amortized in the next twelve months. NU, with its strong credit ratings, has several options available in the financial markets to repay or refinance these maturities with the issuance of new long-term debt. NU, CL&P, NSTAR Electric, and WMECO will reduce their short-term borrowings with cash received from operating cash flows or with the issuance of new long-term debt, as deemed appropriate given capital requirements and maintenance of NU's credit rating and profile. Management expects the future operating cash flows of NU, CL&P, NSTAR Electric and WMECO along with the access to financial markets, will be sufficient to meet any future operating requirements and capital investment forecasted opportunities.
CY2012	us-gaap	Line Of Credit Facility Description LineOfCreditFacilityDescription	CL&P Credit Agreement: On March 26, 2012, CL&P entered into a five-year unsecured revolving credit facility in the amount of $300 million, which expires on March 26, 2017. Under this facility, CL&P can borrow either on a short-term or a long-term basis subject to regulatory approval. As of December 31, 2012, CL&P had $89 million in borrowings outstanding under this credit agreement with a weighted average interest rate of 3.325 percent.
CY2012	nu	Short Term Debt Regulatory Limits ShortTermDebtRegulatoryLimits	The amount of short-term borrowings that may be incurred by CL&P, NSTAR Electric and WMECO is subject to periodic approval by the FERC. On November 30, 2011, the FERC granted authorization to allow CL&P and WMECO to incur total short-term borrowings up to a maximum of $450 million and $300 million, respectively, effective January 1, 2012 through December 31, 2013. On March 22, 2012, the FERC approved CL&P's application requesting to increase its total short-term borrowing capacity from a maximum of $450 million to a maximum of $600 million for the authorization period through December 31, 2013. On May 16, 2012, the FERC granted authorization to allow NSTAR Electric to issue total short-term debt securities in an aggregate principal amount not to exceed $655 million outstanding at any one time, effective October 23, 2012 through October 23, 2014. As a result of the NHPUC having jurisdiction over PSNH's short-term debt, PSNH is not currently required to obtain FERC approval for its short-term borrowings. PSNH is authorized by regulation of the NHPUC to incur short-term borrowings up to 10 percent of net fixed plant plus an additional $60 million until further ordered by the NHPUC. As of December 31, 2012, PSNH's short-term debt authorization under the 10 percent of net fixed plant test plus $60 million totaled approximately $280 million. CL&P's certificate of incorporation contains preferred stock provisions restricting the amount of unsecured debt that CL&P may incur, including limiting unsecured indebtedness with a maturity of less than 10 years to 10 percent of total capitalization. In November 2003, CL&P obtained from its preferred stockholders a waiver of such 10 percent limit for a ten-year period expiring in March 2014, provided that all unsecured indebtedness does not exceed 20 percent of total capitalization. As of December 31, 2012, CL&P had $482 million of unsecured debt capacity available under this authorization.
CY2012	us-gaap	Long Term Debt Description LongTermDebtDescription	On March 22, 2012, NU parent issued $300 million of floating rate Series D Senior Notes with a maturity date of September 20, 2013. The notes have a coupon rate based on the three-month LIBOR rate plus a credit spread of 0.75 percent and will reset quarterly. The notes had an interest rate of 1.059 percent as of December 31, 2012. The proceeds, net of issuance expenses, were used to repay at maturity the NU parent $263 million Series A Senior Notes that matured on April 1, 2012, to repay short-term borrowings outstanding under the NU parent Credit Agreement and for other general corporate purposes.          As of December 31, 2012, amount relates to the purchase price adjustment required to record the NSTAR long-term debt issuances at fair value on the date of the merger. As of December 31, 2011, amount related to a fixed to floating interest rate swap on the $263 million NU parent note that matured on April 1, 2012. The change in fair value of the interest component of the debt was recorded as an adjustment to Current Portion - Long Term Debt as of December 31, 2011 with an equal and offsetting adjustment to Current Derivative Assets. . Fees due to the DOE for the disposal of CL&P's and WMECO's Prior Period Spent Nuclear Fuel and CYAPC's and YAEC's spent nuclear fuel obligation include accumulated interest costs of $350 million and $219.3 million for NU ($177.8 million and $177.6 million for CL&P and $41.7 million and $41.7 million for WMECO) as of December 31, 2012 and 2011, respectively.
CY2012Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	731700000	USD
CY2012Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	576600000	USD
CY2012Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	216700000	USD
CY2012Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	0	USD
CY2012Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive	745000000	USD
CY2012Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal After Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive	4559800000	USD
CY2012Q4	nu	Longtermdebtmaturitiesrepaymentsofprincipaltotal Longtermdebtmaturitiesrepaymentsofprincipaltotal	6829800000	USD
CY2012	nu	Description Of Allocation Of Assets To Subsidiaries DescriptionOfAllocationOfAssetsToSubsidiaries	CL&P, PSNH and WMECO participate in the NUSCO Pension Plan and NUSCO PBOP Plans. Each company participating in the plans is allocated a portion of the total plan assets. As of December 31, 2012 and 2011, the NUSCO Pension Plan has total assets of $2,342.6 million and $2,005.9 million, respectively. CL&P, PSNH and WMECO’s portion of these total plan assets were 40 percent, 17 percent and 9 percent, respectively, as of December 31, 2012, and 43 percent, 14 percent and 10 percent, respectively, as of December 31, 2011. The NUSCO PBOP Plans had total plan assets of $334.9 million and $285.4 million as of December 31, 2012 and 2011, respectively. CL&P, PSNH and WMECO’s share of these assets were 39 percent, 21 percent and 9 percent, respectively, as of both December 31, 2012 and 2011.
CY2012	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	25700000	USD
CY2011	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	17400000	USD
CY2010	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	16100000	USD
CY2011	us-gaap	Employee Stock Ownership Plan Esop Compensation Expense EmployeeStockOwnershipPlanESOPCompensationExpense	8800000	USD
CY2012Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	1545757	shares
CY2009Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	18.96
CY2010Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	18.80
CY2011Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	18.78
CY2010	us-gaap	Employee Stock Ownership Plan Esop Compensation Expense EmployeeStockOwnershipPlanESOPCompensationExpense	8500000	USD
CY2012	us-gaap	Employee Stock Ownership Plan Esop Compensation Expense EmployeeStockOwnershipPlanESOPCompensationExpense	8900000	USD
CY2012Q2	nu	Share Based Compensation Arrangement By Share Based Payment Award Options Converted Nstar Options Upon Merger ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsConvertedNstarOptionsUponMerger	2664894	shares
CY2012Q2	nu	Share Based Compensation Arrangement By Share Based Payment Award Options Converted Nstar Options Upon Merger Total Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsConvertedNstarOptionsUponMergerTotalValue	30.5	USD
CY2012Q2	nu	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nstar Deferred And Performance Awards Converted Upon Merger ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNstarDeferredAndPerformanceAwardsConvertedUponMerger	421775	shares
CY2012Q2	nu	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nstar Deferred And Performance Awards Converted Upon Merger Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNstarDeferredAndPerformanceAwardsConvertedUponMergerValue	15.5	USD
CY2012Q2	nu	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nstar Rsu Retention Awards Converted To Nu Rsus Upon Merger ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNstarRsuRetentionAwardsConvertedToNuRsusUponMerger	195619	shares
CY2012Q2	nu	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nstar Rsu Retention Awards Converted To Nu Rsus Upon Merger Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNstarRsuRetentionAwardsConvertedToNuRsusUponMergerValue	7.2	USD
CY2012Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Authorized ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized	4500000	shares
CY2012Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant	2502512	shares
CY2011Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant	2685615	shares
CY2012	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod	471323	shares
CY2012	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue	30.20
CY2012	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	25800000	USD
CY2011	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	12300000	USD
CY2010	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	10500000	USD
CY2012	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	10200000	USD
CY2011	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	4900000	USD
CY2010	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	4200000	USD
CY2012Q4	us-gaap	Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized	26100000	USD
CY2012	us-gaap	Excess Tax Benefit From Share Based Compensation Financing Activities ExcessTaxBenefitFromShareBasedCompensationFinancingActivities	8500000	USD
CY2011	us-gaap	Excess Tax Benefit From Share Based Compensation Financing Activities ExcessTaxBenefitFromShareBasedCompensationFinancingActivities	1300000	USD
CY2010	us-gaap	Excess Tax Benefit From Share Based Compensation Financing Activities ExcessTaxBenefitFromShareBasedCompensationFinancingActivities	900000	USD
CY2012	nu	Description Of Compensation Cost Not Yet Recognized Period For Recognition DescriptionOfCompensationCostNotYetRecognizedPeriodForRecognition	This cost is expected to be recognized ratably over a weighted-average period of 1.90 years for NU, 2.03 years for CL&P, 1.76 years for NSTAR Electric and 1.84 years for PSNH and WMECO.
CY2009Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	225216	shares
CY2010	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	-112617	shares
CY2010Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	112599	shares
CY2011	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	-65225	shares
CY2011Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	47374	shares
CY2012	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	-1166511	shares
CY2012Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	24.92
CY2010	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	19.12
CY2011	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	18.81
CY2012	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	22.53
CY2010	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	1000000	USD
CY2011	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	1000000	USD
CY2012	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	18700000	USD
CY2012Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	21900000	USD
CY2012Q2	nu	Share Based Compensation Arrangement By Share Based Payment Award Options Converted Nstar Options Upon Merger Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsConvertedNstarOptionsUponMergerWeightedAverageExercisePrice	23.99
CY2012	us-gaap	Employee Service Share Based Compensation Cash Received From Exercise Of Stock Options EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions	26300000	USD
CY2012	us-gaap	Employee Service Share Based Compensation Tax Benefit Realized From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions	7500000	USD
CY2012	us-gaap	Stock Issued During Period Shares Employee Stock Purchase Plans StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans	39422	shares
CY2011	us-gaap	Stock Issued During Period Shares Employee Stock Purchase Plans StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans	35476	shares
CY2011	nu	Espp Discounted Purchase Price First Six Months EsppDiscountedPurchasePriceFirstSixMonths	31.27	USD
CY2012Q4	nu	Employee Stock Purchase Plan Shares Remaining For Future Issuance EmployeeStockPurchasePlanSharesRemainingForFutureIssuance	857280	shares
CY2011Q4	nu	Employee Stock Purchase Plan Shares Remaining For Future Issuance EmployeeStockPurchasePlanSharesRemainingForFutureIssuance	896702	shares
CY2012	nu	Espp Discounted Purchase Price First Six Months EsppDiscountedPurchasePriceFirstSixMonths	33.01	USD
CY2012	nu	Espp Discounted Purchase Price Second Six Months EsppDiscountedPurchasePriceSecondSixMonths	37.89	USD
CY2011	nu	Espp Discounted Purchase Price Second Six Months EsppDiscountedPurchasePriceSecondSixMonths	32.30	USD
CY2012Q4	us-gaap	Deferred Compensation Cashbased Arrangements Liability Classified Noncurrent DeferredCompensationCashbasedArrangementsLiabilityClassifiedNoncurrent	54600000	USD
CY2011Q4	us-gaap	Deferred Compensation Cashbased Arrangements Liability Classified Noncurrent DeferredCompensationCashbasedArrangementsLiabilityClassifiedNoncurrent	52800000	USD
CY2010Q4	us-gaap	Deferred Compensation Cashbased Arrangements Liability Classified Noncurrent DeferredCompensationCashbasedArrangementsLiabilityClassifiedNoncurrent	49900000	USD
CY2012Q4	nu	Deferred Compensation Cash Based Arrangements Expense Pertaining To Deferred Benefits DeferredCompensationCashBasedArrangementsExpensePertainingToDeferredBenefits	4700000	USD
CY2011Q4	nu	Deferred Compensation Cash Based Arrangements Expense Pertaining To Deferred Benefits DeferredCompensationCashBasedArrangementsExpensePertainingToDeferredBenefits	4700000	USD
CY2010Q4	nu	Deferred Compensation Cash Based Arrangements Expense Pertaining To Deferred Benefits DeferredCompensationCashBasedArrangementsExpensePertainingToDeferredBenefits	4200000	USD
CY2012	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	-30900000	USD
CY2011	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	3000000	USD
CY2010	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	9000000	USD
CY2010	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	-6500000	USD
CY2011	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	-26000000	USD
CY2012	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	17600000	USD
CY2012	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	-13300000	USD
CY2011	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	-23000000	USD
CY2010	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	2500000	USD
CY2010	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	201200000	USD
CY2011	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	187700000	USD
CY2012	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	291300000	USD
CY2012	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	800000	USD
CY2011	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	9100000	USD
CY2010	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	9700000	USD
CY2010	us-gaap	Income Tax Reconciliation Tax Credits Investment IncomeTaxReconciliationTaxCreditsInvestment	-3000000	USD
CY2011	us-gaap	Income Tax Reconciliation Tax Credits Investment IncomeTaxReconciliationTaxCreditsInvestment	-2800000	USD
CY2012	us-gaap	Income Tax Reconciliation Tax Credits Investment IncomeTaxReconciliationTaxCreditsInvestment	-3900000	USD
CY2012	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	282800000	USD
CY2012	us-gaap	Income Tax Reconciliation Nondeductible Expense Depreciation IncomeTaxReconciliationNondeductibleExpenseDepreciation	-10800000	USD
CY2012	us-gaap	Income Tax Reconciliation Tax Credits Other IncomeTaxReconciliationTaxCreditsOther	-3800000	USD
CY2012	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	4400000	USD
CY2012	nu	Medicare Subsidy MedicareSubsidy	0	USD
CY2012	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	7600000	USD
CY2012	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-1400000	USD
CY2011	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	200000000	USD
CY2011	us-gaap	Income Tax Reconciliation Nondeductible Expense Depreciation IncomeTaxReconciliationNondeductibleExpenseDepreciation	-14200000	USD
CY2011	us-gaap	Income Tax Reconciliation Tax Credits Other IncomeTaxReconciliationTaxCreditsOther	-3500000	USD
CY2011	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	22100000	USD
CY2011	nu	Medicare Subsidy MedicareSubsidy	0	USD
CY2011	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	-33100000	USD
CY2011	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	2500000	USD
CY2010	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	211600000	USD
CY2010	us-gaap	Income Tax Reconciliation Nondeductible Expense Depreciation IncomeTaxReconciliationNondeductibleExpenseDepreciation	-9500000	USD
CY2010	us-gaap	Income Tax Reconciliation Tax Credits Other IncomeTaxReconciliationTaxCreditsOther	-3800000	USD
CY2010	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	12500000	USD
CY2010	nu	Medicare Subsidy MedicareSubsidy	15600000	USD
CY2010	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	-10500000	USD
CY2010	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-2500000	USD
CY2012	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.340	pure
CY2011	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.299	pure
CY2010	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.348	pure
CY2012Q4	us-gaap	Deferred Tax Assets Derivative Instruments DeferredTaxAssetsDerivativeInstruments	380600000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Derivative Instruments DeferredTaxAssetsDerivativeInstruments	415300000	USD
CY2012Q4	us-gaap	Deferred Tax Assets Regulatory Assets And Liabilities DeferredTaxAssetsRegulatoryAssetsAndLiabilities	257900000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Regulatory Assets And Liabilities DeferredTaxAssetsRegulatoryAssetsAndLiabilities	157900000	USD
CY2012Q4	nu	Deferred Tax Assets Employee Benefits DeferredTaxAssetsEmployeeBenefits	811400000	USD
CY2011Q4	nu	Deferred Tax Assets Employee Benefits DeferredTaxAssetsEmployeeBenefits	539600000	USD
CY2011Q4	nu	Deferred Tax Assets Tax Effect Tax Regulatory Assets DeferredTaxAssetsTaxEffectTaxRegulatoryAssets	15500000	USD
CY2012Q4	nu	Deferred Tax Assets Tax Effect Tax Regulatory Assets DeferredTaxAssetsTaxEffectTaxRegulatoryAssets	17200000	USD
CY2012Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	242400000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	204200000	USD
CY2012Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	4200000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	4600000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	1551900000	USD
CY2012Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2130500000	USD
CY2012Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	3468800000	USD
CY2011Q4	nu	Deferred Tax Liabilities Regulatory Assets Securitized Contract Termination Costs DeferredTaxLiabilitiesRegulatoryAssetsSecuritizedContractTerminationCosts	39600000	USD
CY2012Q4	nu	Deferred Tax Liabilities Regulatory Assets Securitized Contract Termination Costs DeferredTaxLiabilitiesRegulatoryAssetsSecuritizedContractTerminationCosts	16600000	USD
CY2012Q4	nu	Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals DeferredTaxLiabilitiesRegulatoryAssetsOtherRegulatoryDeferrals	1561100000	USD
CY2011Q4	nu	Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals DeferredTaxLiabilitiesRegulatoryAssetsOtherRegulatoryDeferrals	1135000000	USD
CY2011Q4	nu	Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets DeferredTaxLiabilitiesRegulatoryAssetsTaxEffectTaxRegulatoryAssets	184600000	USD
CY2012Q4	nu	Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets DeferredTaxLiabilitiesRegulatoryAssetsTaxEffectTaxRegulatoryAssets	217200000	USD
CY2012Q4	nu	Deferred Tax Liabilities Regulatory Assets Derivative Assets DeferredTaxLiabilitiesRegulatoryAssetsDerivativeAssets	36200000	USD
CY2011Q4	nu	Deferred Tax Liabilities Regulatory Assets Derivative Assets DeferredTaxLiabilitiesRegulatoryAssetsDerivativeAssets	39100000	USD
CY2011Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	24500000	USD
CY2012Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	136100000	USD
CY2012Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	5736500000	USD
CY2011Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	3402200000	USD
CY2012Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	2134700000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	1556500000	USD
CY2012Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Allowance For Doubtful Accounts DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAllowanceForDoubtfulAccounts	64200000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Allowance For Doubtful Accounts DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAllowanceForDoubtfulAccounts	45400000	USD
CY2012Q4	nu	Deferred Tax Liabilities Resulting From Property Tax Accruals And Other DeferredTaxLiabilitiesResultingFromPropertyTaxAccrualsAndOther	89600000	USD
CY2011Q4	nu	Deferred Tax Liabilities Resulting From Property Tax Accruals And Other DeferredTaxLiabilitiesResultingFromPropertyTaxAccrualsAndOther	58900000	USD
CY2011Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	1920500000	USD
CY2012Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	214600000	USD
CY2011Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	178600000	USD
CY2012Q4	nu	Purchase Accounting Deferred Tax Asset PurchaseAccountingDeferredTaxAsset	146400000	USD
CY2011Q4	nu	Purchase Accounting Deferred Tax Asset PurchaseAccountingDeferredTaxAsset	0	USD
CY2012Q4	nu	Goodwill Deferred Tax Liability GoodwillDeferredTaxLiability	210900000	USD
CY2011Q4	nu	Goodwill Deferred Tax Liability GoodwillDeferredTaxLiability	0	USD
CY2012Q4	us-gaap	Tax Credit Carryforward Amount TaxCreditCarryforwardAmount	3800000	USD
CY2012Q4	us-gaap	Operating Loss Carryforwards OperatingLossCarryforwards	606900000	USD
CY2011Q4	us-gaap	Operating Loss Carryforwards OperatingLossCarryforwards	510200000	USD
CY2011Q4	us-gaap	Tax Credit Carryforward Amount TaxCreditCarryforwardAmount	6600000	USD
CY2012Q4	us-gaap	Operating Loss Carryforwards Valuation Allowance OperatingLossCarryforwardsValuationAllowance	300000	USD
CY2009Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	124300000	USD
CY2010	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	10800000	USD
CY2010	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	-34300000	USD
CY2010	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	-400000	USD
CY2010Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	101200000	USD
CY2011	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	8000000	USD
CY2011Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	73500000	USD
CY2012	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	10300000	USD
CY2012Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	83100000	USD
CY2010	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	800000	USD
CY2012	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	100000	USD
CY2012	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	-800000	USD
CY2011	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	-35700000	USD
CY2012	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Expense UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense	3100000	USD
CY2011	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Expense UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense	-2800000	USD
CY2010	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Expense UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense	-24800000	USD
CY2012Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	10100000	USD
CY2011Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	7100000	USD
CY2012Q4	nu	State Tax Settlements In2011 StateTaxSettlementsIn2011	29100000	USD
CY2012Q4	nu	State Tax Settlements Tax Expense Reductions In2011 StateTaxSettlementsTaxExpenseReductionsIn2011	22400000	USD
CY2012Q4	nu	State Tax Settlements In2010 StateTaxSettlementsIn2010	35000000	USD
CY2012Q4	nu	State Tax Settlements Tax Expense Reductions In2010 StateTaxSettlementsTaxExpenseReductionsIn2010	6000000	USD
CY2012	us-gaap	Significant Change In Unrecognized Tax Benefits Is Reasonably Possible Other Information SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleOtherInformation	These estimated changes could have an impact on NU's and CL&P's 2013 earnings of zero to $6 million and zero to $16 million, respectively.  Other companies’ impacts are not expected to be material.
CY2012Q4	nu	Nstar Electric2011 Federal Tax Refund NstarElectric2011FederalTaxRefund	166800000	USD
CY2012Q4	us-gaap	Significant Change In Unrecognized Tax Benefits Is Reasonably Possible Estimated Range Of Change Upper Bound SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeUpperBound	50000000	USD
CY2010Q4	us-gaap	Accrual For Environmental Loss Contingencies AccrualForEnvironmentalLossContingencies	37100000	USD
CY2011	us-gaap	Accrual For Environmental Loss Contingencies Provision For New Losses AccrualForEnvironmentalLossContingenciesProvisionForNewLosses	1600000	USD
CY2011	us-gaap	Accrual For Environmental Loss Contingencies Payments AccrualForEnvironmentalLossContingenciesPayments	7000000	USD
CY2011Q4	us-gaap	Accrual For Environmental Loss Contingencies AccrualForEnvironmentalLossContingencies	31700000	USD
CY2012	nu	Environmental Liability Assumed In Merger EnvironmentalLiabilityAssumedInMerger	11800000	USD
CY2012	us-gaap	Accrual For Environmental Loss Contingencies Provision For New Losses AccrualForEnvironmentalLossContingenciesProvisionForNewLosses	4700000	USD
CY2012	us-gaap	Accrual For Environmental Loss Contingencies Payments AccrualForEnvironmentalLossContingenciesPayments	8800000	USD
CY2012Q4	us-gaap	Accrual For Environmental Loss Contingencies AccrualForEnvironmentalLossContingencies	39400000	USD
CY2012	nu	Environmental Site Quantity EnvironmentalSiteQuantity	77	pure
CY2011	nu	Environmental Site Quantity EnvironmentalSiteQuantity	59	pure
CY2012Q4	us-gaap	Accrual For Environmental Loss Contingencies Component Amount AccrualForEnvironmentalLossContingenciesComponentAmount	2100000	USD
CY2012	nu	Cumulative Environmental Remediation Expense CumulativeEnvironmentalRemediationExpense	19500000	USD
CY2012Q4	nu	Superfund Site Information SuperfundSiteInformation	CERCLA: The federal Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA) and its amendments or state equivalents impose joint and several strict liabilities, regardless of fault, upon generators of hazardous substances resulting in removal and remediation costs and environmental damages.  Liabilities under these laws can be material and in some instances may be imposed without regard to fault or for past acts that may have been lawful at the time they occurred.  Of the total sites included in the remediation and long-term monitoring phase, 10 sites (2 for CL&P, 4 for NSTAR Electric, 4 for PSNH and 1 for WMECO) are superfund sites under CERCLA for which the Company has been notified that it is a potentially responsible party but for which the site assessment and remediation are not being managed by the Company.  As of December 31, 2012, a liability of $1 million ($0.4 million for CL&P, $0.1 million for NSTAR Electric and $0.4 million for PSNH) accrued on these sites represents management's best estimate of its potential remediation costs with respect to these superfund sites.
CY2012	nu	Cumulative Payments Environmental Loss Contingencies CumulativePaymentsEnvironmentalLossContingencies	17400000	USD
CY2012Q4	nu	Range Of Potential Loss On Enivornmental Sites RangeOfPotentialLossOnEnivornmentalSites	As of December 31, 2012, for 8 environmental sites (3 for CL&P, 2 for PSNH, and 1 for WMECO) that are included in the Company's reserve for environmental costs, the information known and nature of the remediation options at those sites allow for the Company to estimate the range of losses for environmental costs. As of December 31, 2012, $2.6 million ($0.6 million for CL&P and $0.7 million for PSNH) had been accrued as a liability for these sites, which represent management's best estimates of the liabilities for environmental costs. These amounts are the best estimates with estimated ranges of additional losses from zero million to $15.2 million (zero to $1.3 million for CL&P, zero to $4.1 million for PSNH, and zero to $8.6 million for WMECO). As of December 31, 2012, for 23 environmental sites (5 for CL&P,1 for NSTAR Electric, 4 for PSNH, and 3 for WMECO) included in the Company’s reserve for environmental costs, management cannot reasonably estimate the exposure to loss in excess of the reserve, or range of loss, as these sites are under investigation and/or there is significant uncertainty as to what remedial actions, if any, the Company may be required to undertake. As of December 31, 2012, $17 million ($1.7 million for CL&P, $0.2 million for PSNH, and $0.5 million for WMECO) had been accrued as a liability for these sites. As of December 31, 2012, for the remaining 46 environmental sites (11 for CL&P, 15 for NSTAR Electric, 10 for PSNH, and 2 for WMECO) included in the Company’s reserve for environmental costs, the $19.8 million accrual ($1.4 million for CL&P, $1.7 million for NSTAR Electric, $4 million for PSNH, and $0.1 million for WMECO) represents management’s best estimate of the liability and no additional loss is anticipated.
CY2012Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Balance On Third Anniversary UnrecordedUnconditionalPurchaseObligationBalanceOnThirdAnniversary	572800000	USD
CY2012Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Balance On Fourth Anniversary UnrecordedUnconditionalPurchaseObligationBalanceOnFourthAnniversary	501900000	USD
CY2012Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Balance On Fifth Anniversary UnrecordedUnconditionalPurchaseObligationBalanceOnFifthAnniversary	432900000	USD
CY2012Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Due After Five Years UnrecordedUnconditionalPurchaseObligationDueAfterFiveYears	2897500000	USD
CY2012Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Balance Sheet Amount UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount	5806700000	USD
CY2012Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Balance On First Anniversary UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary	717700000	USD
CY2012Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Balance On Second Anniversary UnrecordedUnconditionalPurchaseObligationBalanceOnSecondAnniversary	683900000	USD
CY2012	nu	Supply And Stranded Costs Obligations Incurred SupplyAndStrandedCostsObligationsIncurred	216800000	USD
CY2011	nu	Supply And Stranded Costs Obligations Incurred SupplyAndStrandedCostsObligationsIncurred	156000000	USD
CY2012	nu	Peaker Cfd Expense Incurred PeakerCfdExpenseIncurred	59300000	USD
CY2011	nu	Peaker Cfd Expense Incurred PeakerCfdExpenseIncurred	40200000	USD
CY2012	nu	Natural Gas Procurement Costs Incurred NaturalGasProcurementCostsIncurred	243100000	USD
CY2011	nu	Natural Gas Procurement Costs Incurred NaturalGasProcurementCostsIncurred	191700000	USD
CY2010	nu	Fuel Costs Incurred FuelCostsIncurred	171100000	USD
CY2011	nu	Fuel Costs Incurred FuelCostsIncurred	113200000	USD
CY2012	nu	Transmission Support Commitment Costs Incurred TransmissionSupportCommitmentCostsIncurred	24800000	USD
CY2011	nu	Transmission Support Commitment Costs Incurred TransmissionSupportCommitmentCostsIncurred	18100000	USD
CY2012	nu	Renewable Energy Costs Incurred RenewableEnergyCostsIncurred	48700000	USD
CY2011	nu	Renewable Energy Costs Incurred RenewableEnergyCostsIncurred	5100000	USD
CY2012Q4	nu	Court Of Federal Claims Ruling To Cyapc CourtOfFederalClaimsRulingToCyapc	34200000	USD
CY2012Q4	nu	Court Of Federal Claims Ruling To Yaec CourtOfFederalClaimsRulingToYaec	32900000	USD
CY2012Q4	nu	Court Of Federal Claims Ruling To Myapc CourtOfFederalClaimsRulingToMyapc	75800000	USD
CY2012Q4	nu	Trial Court Ruling To Cyapc TrialCourtRulingToCyapc	39700000	USD
CY2012Q4	nu	Trial Court Ruling To Yaec TrialCourtRulingToYaec	21200000	USD
CY2012Q4	nu	Trial Court Ruling To Myapc TrialCourtRulingToMyapc	81700000	USD
CY2012Q4	nu	Receivable From Doe At Year End2012 ReceivableFromDoeAtYearEnd2012	77900000	USD
CY2012Q4	nu	Connecticut Yankee Proceeds Received ConnecticutYankeeProceedsReceived	39600000	USD
CY2012Q4	nu	Yankee Atomic Proceeds Received YankeeAtomicProceedsReceived	38300000	USD
CY2012Q4	nu	Maine Yankee Proceeds Received MaineYankeeProceedsReceived	81700000	USD
CY2012Q4	nu	Nstar Electric Dpu Storm Penalty NstarElectricDpuStormPenalty	4100000	USD
CY2012Q4	nu	Wmeco Dpu Storm Penalty WmecoDpuStormPenalty	2000000	USD
CY2012Q4	nu	Cumulative Cpsl Costs2006 To2011 CumulativeCpslCosts2006To2011	83000000	USD
CY2012Q4	nu	Cpsl Revenue Requirement For2012 To2015 CpslRevenueRequirementFor2012To2015	15000000	USD
CY2012Q4	nu	Bad Debt Service Adder As Of2011 BadDebtServiceAdderAsOf2011	34000000	USD
CY2012Q4	nu	Pre Tax Nstar Electric Bad Debt Adder Reserve Recorded In2012 PreTaxNstarElectricBadDebtAdderReserveRecordedIn2012	28000000	USD
CY2012Q4	nu	Post Tax Nstar Electric Bad Debt Adder Reserve In2012 PostTaxNstarElectricBadDebtAdderReserveIn2012	17000000	USD
CY2012	us-gaap	Lease And Rental Expense LeaseAndRentalExpense	14800000	USD
CY2011	us-gaap	Lease And Rental Expense LeaseAndRentalExpense	8400000	USD
CY2010	us-gaap	Lease And Rental Expense LeaseAndRentalExpense	11900000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Due Current CapitalLeasesFutureMinimumPaymentsDueCurrent	2800000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Due In Two Years CapitalLeasesFutureMinimumPaymentsDueInTwoYears	2200000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Due In Three Years CapitalLeasesFutureMinimumPaymentsDueInThreeYears	2200000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Due In Four Years CapitalLeasesFutureMinimumPaymentsDueInFourYears	2000000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Due In Five Years CapitalLeasesFutureMinimumPaymentsDueInFiveYears	2000000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Due Thereafter CapitalLeasesFutureMinimumPaymentsDueThereafter	7500000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Due CapitalLeasesFutureMinimumPaymentsDue	18700000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Interest Included In Payments CapitalLeasesFutureMinimumPaymentsInterestIncludedInPayments	7600000	USD
CY2012Q4	us-gaap	Capital Leases Future Minimum Payments Present Value Of Net Minimum Payments CapitalLeasesFutureMinimumPaymentsPresentValueOfNetMinimumPayments	11100000	USD
CY2012Q4	us-gaap	Operating Leases Future Minimum Payments Due Current OperatingLeasesFutureMinimumPaymentsDueCurrent	22400000	USD
CY2012Q4	us-gaap	Operating Leases Future Minimum Payments Due In Two Years OperatingLeasesFutureMinimumPaymentsDueInTwoYears	16600000	USD
CY2012Q4	us-gaap	Operating Leases Future Minimum Payments Due In Three Years OperatingLeasesFutureMinimumPaymentsDueInThreeYears	14100000	USD
CY2012Q4	us-gaap	Operating Leases Future Minimum Payments Due In Four Years OperatingLeasesFutureMinimumPaymentsDueInFourYears	11200000	USD
CY2012Q4	us-gaap	Operating Leases Future Minimum Payments Due In Five Years OperatingLeasesFutureMinimumPaymentsDueInFiveYears	8600000	USD
CY2012Q4	us-gaap	Operating Leases Future Minimum Payments Due Thereafter OperatingLeasesFutureMinimumPaymentsDueThereafter	23300000	USD
CY2012Q4	us-gaap	Operating Leases Future Minimum Payments Due OperatingLeasesFutureMinimumPaymentsDue	96200000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax	100000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax	-2700000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax	-7900000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax	0	USD
CY2011Q4	us-gaap	Operating Income Loss OperatingIncomeLoss	184900000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Tax Portion Attributable To Parent1 OtherComprehensiveIncomeLossTaxPortionAttributableToParent1	-1300000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Tax Portion Attributable To Parent1 OtherComprehensiveIncomeLossTaxPortionAttributableToParent1	-17000000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Tax Portion Attributable To Parent1 OtherComprehensiveIncomeLossTaxPortionAttributableToParent1	400000	USD
CY2011Q4	us-gaap	Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	1100000	USD
CY2011Q4	us-gaap	Accumulated Other Comprehensive Income Loss Defined Benefit Pension And Other Postretirement Plans Net Of Tax AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax	53400000	USD
CY2012Q4	us-gaap	Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	1300000	USD
CY2012Q4	us-gaap	Accumulated Other Comprehensive Income Loss Defined Benefit Pension And Other Postretirement Plans Net Of Tax AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax	57800000	USD
CY2012Q4	us-gaap	Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax	-16400000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment On Derivatives Included In Net Income Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentOnDerivativesIncludedInNetIncomeNetOfTax	-700000	USD
CY2011	us-gaap	Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax	-14900000	USD
CY2011Q4	us-gaap	Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax	-18400000	USD
CY2010Q4	us-gaap	Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax	-4200000	USD
CY2012	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesTax	1300000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesTax	-9500000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesTax	200000	USD
CY2010	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax	200000	USD
CY2011	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax	400000	USD
CY2012	nu	Pension Serp And Pbop Gain Loss To Be Reclassified Within Twelve Months PensionSerpAndPbopGainLossToBeReclassifiedWithinTwelveMonths	-10500000	USD
CY2012	us-gaap	Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths	-2000000	USD
CY2012	us-gaap	Line Of Credit Facility Dividend Restrictions LineOfCreditFacilityDividendRestrictions	CL&P, NSTAR Electric, PSNH and WMECO are subject to Section 305 of the Federal Power Act that makes it unlawful for a public utility to make or pay a dividend from any funds "properly included in its capital account." Management believes that this Federal Power Act restriction, as applied to CL&P, NSTAR Electric, PSNH and WMECO, would not be construed or applied by the FERC to prohibit the payment of dividends for lawful and legitimate business purposes from retained earnings. In addition, certain state statutes may impose additional limitations on such companies and on Yankee Gas and NSTAR Gas. Such state law restrictions do not restrict payment of dividends from retained earnings or net income. CL&P, NSTAR Electric, PSNH, WMECO, Yankee Gas and NSTAR Gas also have revolving credit agreements that impose leverage restrictions including consolidated total debt to total capitalization ratio requirements. The Retained Earnings balances subject to these leverage restrictions are $1.803 billion for NU, $839.6 million for CL&P, $1.210 billion for NSTAR Electric, $395.1 million for PSNH and $160.6 million for WMECO as of December 31, 2012.
CY2012Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	5
CY2011Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	196052770	shares
CY2012Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	332509383	shares
CY2011Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	380000000	shares
CY2012Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	380000000	shares
CY2012Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	18455749	shares
CY2011Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	18894078	shares
CY2012Q4	us-gaap	Preferred Stock Value Outstanding PreferredStockValueOutstanding	155600000	USD
CY2011Q4	us-gaap	Preferred Stock Value Outstanding PreferredStockValueOutstanding	116200000	USD
CY2012	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	783812	shares
CY2011	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	394401	shares
CY2010	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	249301	shares
CY2012	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.90
CY2011	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.22
CY2010	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.20
CY2012	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.89
CY2011	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.22
CY2010	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.19
CY2012	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	4266	shares
CY2011	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	4314	shares
CY2010	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	1578	shares
CY2012	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	740800000	USD
CY2012	us-gaap	Utilities Operating Expense Other UtilitiesOperatingExpenseOther	4414800000	USD
CY2011	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	463200000	USD
CY2011	us-gaap	Utilities Operating Expense Other UtilitiesOperatingExpenseOther	3208300000	USD
CY2010	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	623700000	USD
CY2010	us-gaap	Utilities Operating Expense Other UtilitiesOperatingExpenseOther	3474600000	USD
CY2012Q4	us-gaap	Revenues Revenues	1684000000	USD
CY2012Q4	us-gaap	Operating Income Loss OperatingIncomeLoss	331400000	USD
CY2012Q4	us-gaap	Net Income Loss NetIncomeLoss	176600000	USD
CY2011Q1	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.64
CY2012	us-gaap	Subsequent Event Description SubsequentEventDescription	See Note 9, "Long-Term Debt" to the consolidated financial statements for information regarding the January 2013 long-term debt issuance for CL&P. See Note 12C, "Commitments and Contingencies – Deferred Contractual Obligations" to the consolidated financial statements for information regarding the receipt of the DOE proceeds by the Yankee Companies in January 2013. See Note 11, "Income Taxes," for discussion of the federal legislation enacted on January 2, 2013. On February 8, 2013, a blizzard caused damage to the electric delivery systems of CL&P and NSTAR Electric. Management believes that this storm will cost between $100 million to $120 million, with approximately 90 percent of those costs relating to NSTAR Electric. Management expects the costs to meet the criteria for specific cost recovery in Connecticut and Massachusetts and, as a result, does not expect the storm to have a material impact on the results of operations of CL&P or NSTAR Electric. Each operating company will seek recovery of these anticipated deferred storm costs through its applicable regulatory recovery process.
CY2011Q2	us-gaap	Revenues Revenues	1047500000	USD
CY2011Q3	us-gaap	Revenues Revenues	1114900000	USD
CY2011Q3	us-gaap	Operating Income Loss OperatingIncomeLoss	203800000	USD
CY2011Q2	us-gaap	Operating Income Loss OperatingIncomeLoss	178100000	USD
CY2011Q2	us-gaap	Net Income Loss NetIncomeLoss	78700000	USD
CY2011Q3	us-gaap	Net Income Loss NetIncomeLoss	91400000	USD
CY2011Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	90000000	USD
CY2011Q2	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	77300000	USD
CY2012Q1	us-gaap	Revenues Revenues	1099600000	USD
CY2012Q1	us-gaap	Operating Income Loss OperatingIncomeLoss	214400000	USD
CY2012Q1	us-gaap	Net Income Loss NetIncomeLoss	100800000	USD
CY2011Q1	us-gaap	Revenues Revenues	1235300000	USD
CY2011Q1	us-gaap	Operating Income Loss OperatingIncomeLoss	227400000	USD
CY2011Q1	us-gaap	Net Income Loss NetIncomeLoss	115600000	USD
CY2011Q1	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	114200000	USD
CY2012Q1	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	99300000	USD
CY2012Q4	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	174700000	USD
CY2011Q4	us-gaap	Revenues Revenues	1068000000	USD
CY2011Q4	us-gaap	Net Income Loss NetIncomeLoss	114800000	USD
CY2011Q4	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	113200000	USD
CY2012Q1	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.56
CY2012Q2	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.15
CY2012Q3	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.66
CY2012Q4	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.55
CY2011Q4	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.64
CY2011Q3	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.51
CY2011Q2	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	0.44
CY2012	us-gaap	Valuation Allowances And Reserves Charged To Cost And Expense ValuationAllowancesAndReservesChargedToCostAndExpense	36275000	USD
CY2011	us-gaap	Valuation Allowances And Reserves Charged To Cost And Expense ValuationAllowancesAndReservesChargedToCostAndExpense	16420000	USD
CY2010	us-gaap	Valuation Allowances And Reserves Charged To Cost And Expense ValuationAllowancesAndReservesChargedToCostAndExpense	31352000	USD
CY2012	us-gaap	Valuation Allowances And Reserves Deductions ValuationAllowancesAndReservesDeductions	80462000	USD
CY2011	us-gaap	Valuation Allowances And Reserves Deductions ValuationAllowancesAndReservesDeductions	60584000	USD
CY2010	us-gaap	Valuation Allowances And Reserves Deductions ValuationAllowancesAndReservesDeductions	85632000	USD
CY2009Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	127517000	USD
CY2010Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	119190000	USD
CY2011Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	115689000	USD
CY2012Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	165549000	USD
CY2012	us-gaap	Valuation Allowances And Reserves Charged To Other Accounts ValuationAllowancesAndReservesChargedToOtherAccounts	34761000	USD
CY2011	us-gaap	Valuation Allowances And Reserves Charged To Other Accounts ValuationAllowancesAndReservesChargedToOtherAccounts	40663000	USD
CY2010	us-gaap	Valuation Allowances And Reserves Charged To Other Accounts ValuationAllowancesAndReservesChargedToOtherAccounts	45953000	USD
CY2012Q4	nu	Impact Related To Nstar Merger ImpactRelatedToNstarMerger	59286000	USD
CY2011Q4	nu	Uncollectible Hardship Accounts Receivable Reserves UncollectibleHardshipAccountsReceivableReserves	68600000	USD
CY2010Q4	nu	Uncollectible Hardship Accounts Receivable Reserves UncollectibleHardshipAccountsReceivableReserves	65000000	USD
CY2012	dei	Entity Registrant Name EntityRegistrantName	NORTHEAST UTILITIES
CY2012	dei	Document Type DocumentType	10-K
CY2012	dei	Document Period End Date DocumentPeriodEndDate	2012-12-31
CY2012	dei	Entity Central Index Key EntityCentralIndexKey	0000072741
CY2012	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2013Q1	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	314338271	shares
CY2012Q2	dei	Entity Public Float EntityPublicFloat	12177646948	USD
CY2012	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2012	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2012	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2012	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2012	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2012
CY2012	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2012	dei	Amendment Flag AmendmentFlag	false
CY2010	us-gaap	Pension And Other Postretirement Benefit Expense PensionAndOtherPostretirementBenefitExpense	103861000	USD
CY2012	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	17032000	USD
CY2012Q2	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	44300000	USD
CY2012Q2	us-gaap	Net Income Loss NetIncomeLoss	46200000	USD
CY2010	nu	Supply And Stranded Costs Obligations Incurred SupplyAndStrandedCostsObligationsIncurred	196200000	USD
CY2012Q2	us-gaap	Revenues Revenues	1628700000	USD
CY2012Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	207600000	USD

Files In Submission

Name	View Source	Status
0000072741-13-000015-index-headers.html	Edgar Link	pending
0000072741-13-000015-index.html	Edgar Link	pending
0000072741-13-000015.txt	Edgar Link	pending
0000072741-13-000015-xbrl.zip	Edgar Link	pending
clpexhibit12.htm	Edgar Link	pending
clpexhibit31.htm	Edgar Link	pending
clpexhibit311.htm	Edgar Link	pending
clpexhibit32.htm	Edgar Link	pending
correspondence002.gif	Edgar Link	pending
exhibit104.htm	Edgar Link	pending
exhibit21.htm	Edgar Link	pending
exhibit231.htm	Edgar Link	pending
exhibit232.htm	Edgar Link	pending
f2012form10kedgar.htm	Edgar Link	pending
f2012form10kedgar002.gif	Edgar Link	pending
FilingSummary.xml	Edgar Link	unprocessable
Financial_Report.xls	Edgar Link	pending
nstarexhibit12.htm	Edgar Link	pending
nstarexhibit31.htm	Edgar Link	pending
nstarexhibit311.htm	Edgar Link	pending
nstarexhibit32.htm	Edgar Link	pending
nu-20121231.xml	Edgar Link	completed
nu-20121231.xsd	Edgar Link	pending
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nu-20121231_def.xml	Edgar Link	unprocessable
nu-20121231_lab.xml	Edgar Link	unprocessable
nu-20121231_pre.xml	Edgar Link	unprocessable
nuexhibit12.htm	Edgar Link	pending
nuexhibit31.htm	Edgar Link	pending
nuexhibit311.htm	Edgar Link	pending
nuexhibit32.htm	Edgar Link	pending
psnhexhibit12.htm	Edgar Link	pending
psnhexhibit31.htm	Edgar Link	pending
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report.css	Edgar Link	pending
Show.js	Edgar Link	pending
wmecoexhibit12.htm	Edgar Link	pending
wmecoexhibit31.htm	Edgar Link	pending
wmecoexhibit311.htm	Edgar Link	pending
wmecoexhibit32.htm	Edgar Link	pending

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$ES
Eversource Energy Stock $es

2012 Q3 Form 10-K Financial Statement

#000007274113000015 Filed on February 28, 2013

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$ES Eversource Energy Stock $es

2012 Q3 Form 10-K Financial Statement

#000007274113000015 Filed on February 28, 2013

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$ES
Eversource Energy Stock $es