$BIIB
BIOGEN INC. Stock $biib

$biib Overview Financials Valuation SEC Filings Company Website

2019 Q4 Form 10-K Financial Statement

#000087504520000005 Filed on February 06, 2020

Income Statement

Concept	2019 Q4	2019	2018 Q4
Revenue	$3.671B	$14.38B	$3.526B
YoY Change	4.11%	6.88%	6.63%
Cost Of Revenue	$447.1M	$1.955B	$488.5M
YoY Change	-8.47%	7.66%	-4.07%
Gross Profit	$3.224B	$12.42B	$3.038B
YoY Change	6.14%	6.75%	8.58%
Gross Profit Margin	87.82%	86.4%	86.15%
Selling, General & Admin	$661.0M	$2.375B	$591.1M
YoY Change	11.83%	12.74%	3.27%
% of Gross Profit	20.5%	19.12%	19.46%
Research & Development	$689.3M	$2.281B	$611.6M
YoY Change	12.7%	-12.19%	4.08%
% of Gross Profit	21.38%	18.36%	20.13%
Depreciation & Amortization	$113.0M	$464.7M	$329.7M
YoY Change	-65.73%	-28.56%	270.45%
% of Gross Profit	3.5%	3.74%	10.85%
Operating Expenses	$1.427B	$4.655B	$1.359B
YoY Change	4.97%	-1.02%	4.46%
Operating Profit	$1.797B	$7.043B	$1.679B
YoY Change	7.08%	19.6%	12.16%
Interest Expense	-$72.70M	$187.4M	-$82.70M
YoY Change	-12.09%	-6.58%	-6.87%
% of Operating Profit	-4.04%	2.66%	-4.93%
Other Income/Expense, Net	-$36.40M	$83.30M	-$1.700M
YoY Change	2041.18%	657.27%	-75.71%
Pretax Income	$1.728B	$4.725B	$1.415B
YoY Change	22.19%	21.88%	1.05%
Income Tax	$276.1M	$1.158B	$445.9M
% Of Pretax Income	15.97%	24.51%	31.52%
Net Earnings	$1.440B	$5.889B	$946.8M
YoY Change	52.06%	32.9%	-418.36%
Net Earnings / Revenue	39.21%	40.96%	26.85%
Basic Earnings Per Share	$8.10	$31.47	$4.74
Diluted Earnings Per Share	$8.08	$31.42	$4.73
COMMON SHARES
Basic Shares Outstanding	177.8M shares	187.1M shares	199.8M shares
Diluted Shares Outstanding	178.2M shares	187.4M shares	200.3M shares

Balance Sheet

Concept	2019 Q4	2019	2018 Q4
SHORT-TERM ASSETS
Cash & Short-Term Investments	$4.476B	$4.476B	$3.538B
YoY Change	26.51%	26.51%	-4.09%
Cash & Equivalents	$2.914B	$2.914B	$1.225B
Short-Term Investments	$1.562B	$1.562B	$2.313B
Other Short-Term Assets	$631.0M	$631.0M	$687.6M
YoY Change	-8.23%	-8.28%	-28.52%
Inventory	$804.2M	$804.0M	$929.9M
Prepaid Expenses
Receivables	$1.881B	$2.471B	$1.959B
Other Receivables	$0.00	$0.00	$0.00
Total Short-Term Assets	$8.382B	$8.382B	$7.641B
YoY Change	9.7%	9.7%	-2.95%
LONG-TERM ASSETS
Property, Plant & Equipment	$3.247B	$3.674B	$3.601B
YoY Change	-9.83%	2.03%	13.16%
Goodwill	$5.758B		$5.706B
YoY Change	0.9%		23.18%
Intangibles	$3.527B		$3.120B
YoY Change	13.06%		-19.58%
Long-Term Investments	$2.654B	$2.654B	$3.039B
YoY Change	-12.67%	-12.67%	-25.07%
Other Assets	$1.253B	$3.239B	$1.691B
YoY Change	-25.9%	48.44%	291.71%
Total Long-Term Assets	$18.85B	$18.85B	$17.65B
YoY Change	6.83%	6.82%	11.84%
TOTAL ASSETS
Total Short-Term Assets	$8.382B	$8.382B	$7.641B
Total Long-Term Assets	$18.85B	$18.85B	$17.65B
Total Assets	$27.23B	$27.23B	$25.29B
YoY Change	7.69%	7.69%	6.92%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$530.8M	$531.0M	$370.5M
YoY Change	43.27%	43.13%	-6.32%
Accrued Expenses	$1.683B	$1.683B	$1.546B
YoY Change	8.86%	8.86%	25.08%
Deferred Revenue
YoY Change
Short-Term Debt	$0.00	$0.00	$0.00
YoY Change
Long-Term Debt Due	$1.496B	$1.496B	$0.00
YoY Change			-100.0%
Total Short-Term Liabilities	$4.864B	$4.864B	$3.295B
YoY Change	47.6%	47.62%	-2.17%
LONG-TERM LIABILITIES
Long-Term Debt	$4.459B	$4.459B	$5.937B
YoY Change	-24.89%	-24.89%	0.03%
Other Long-Term Liabilities	$1.349B	$1.762B	$1.389B
YoY Change	-2.91%	26.85%	-14.69%
Total Long-Term Liabilities	$1.349B	$6.221B	$1.389B
YoY Change	-2.91%	-15.08%	-14.69%
TOTAL LIABILITIES
Total Short-Term Liabilities	$4.864B	$4.864B	$3.295B
Total Long-Term Liabilities	$1.349B	$6.221B	$1.389B
Total Liabilities	$13.90B	$13.89B	$12.26B
YoY Change	13.36%	13.41%	10.88%
SHAREHOLDERS EQUITY
Retained Earnings	$16.46B		$16.26B
YoY Change	1.22%		2.82%
Common Stock	$100.0K		$100.0K
YoY Change	0.0%		0.0%
Preferred Stock
YoY Change
Treasury Stock (at cost)	$2.977B		$2.977B
YoY Change	0.0%		0.0%
Treasury Stock Shares	23.80M shares		23.80M shares
Shareholders Equity	$13.34B	$13.34B	$13.04B
YoY Change
Total Liabilities & Shareholders Equity	$27.23B	$27.23B	$25.29B
YoY Change	7.69%	7.69%	6.92%

Cashflow Statement

Concept	2019 Q4	2019	2018 Q4
OPERATING ACTIVITIES
Net Income	$1.440B	$5.889B	$946.8M
YoY Change	52.06%	32.9%	-418.36%
Depreciation, Depletion And Amortization	$113.0M	$464.7M	$329.7M
YoY Change	-65.73%	-28.56%	270.45%
Cash From Operating Activities	$1.960B	$7.079B	$1.895B
YoY Change	3.42%	14.4%	24.84%
INVESTING ACTIVITIES
Capital Expenditures	-$265.4M	$514.5M	-$225.9M
YoY Change	17.49%	-33.23%	-23.58%
Acquisitions		$744.4M
YoY Change
Other Investing Activities	$973.8M	-$27.00M	-$1.399B
YoY Change	-169.59%	6650.0%	195.29%
Cash From Investing Activities	$708.4M	$470.5M	-$1.625B
YoY Change	-143.59%	-122.99%	111.22%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net		$5.868B
YoY Change		34.82%
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	-2.116B	-$5.860B	-1.437B
YoY Change	47.28%	31.05%	102.94%
NET CHANGE
Cash From Operating Activities	1.960B	$7.079B	1.895B
Cash From Investing Activities	708.4M	$470.5M	-1.625B
Cash From Financing Activities	-2.116B	-$5.860B	-1.437B
Net Change In Cash	552.2M	$1.689B	-1.167B
YoY Change	-147.32%	-610.8%	-2967.08%
FREE CASH FLOW
Cash From Operating Activities	$1.960B	$7.079B	$1.895B
Capital Expenditures	-$265.4M	$514.5M	-$225.9M
Free Cash Flow	$2.226B	$6.564B	$2.121B
YoY Change	4.92%	21.17%	16.95%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2019	biib	Compensation Expense Over Purchase Period CompensationExpenseOverPurchasePeriod	The fair value of the look-back provision plus the 15% discount
CY2019	biib	Estimated Useful Lives Of Leasehold Improvements EstimatedUsefulLivesOfLeaseholdImprovements	Lesser of the useful life or the term of the respective lease
CY2019	biib	Payment Terms Of Accounts Receivable PaymentTermsOfAccountsReceivable	30 to 90 days
CY2019	biib	Purchase Price Of Common Stock Under Espp PurchasePriceOfCommonStockUnderEspp	85% of the lower of (i) the fair market value per share of the common stock on the first business day of an offering period and (ii) the fair market value per share of the common stock on the purchase date
CY2019	dei	Amendment Flag AmendmentFlag	false
CY2019	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2019	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2019	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2019
CY2019	dei	Entity Central Index Key EntityCentralIndexKey	0000875045
CY2018Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0005
CY2019Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0005
CY2018Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.001
CY2019Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.001
CY2018	us-gaap	Servicing Asset At Fair Value Description Of Changes In Valuation Inputs ServicingAssetAtFairValueDescriptionOfChangesInValuationInputs	0
CY2017Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant	0	shares
CY2018Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant	0	shares
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant	0	shares
CY2018Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	23800000	shares
CY2019Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	23800000	shares
CY2019	dei	Document Type DocumentType	10-K
CY2019	dei	Document Annual Report DocumentAnnualReport	true
CY2019	dei	Document Period End Date DocumentPeriodEndDate	2019-12-31
CY2019	dei	Document Transition Report DocumentTransitionReport	false
CY2019	dei	Entity File Number EntityFileNumber	0-19311
CY2019	dei	Entity Registrant Name EntityRegistrantName	BIOGEN INC.
CY2019	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
CY2019	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	33-0112644
CY2019	dei	Entity Address Address Line1 EntityAddressAddressLine1	225 Binney Street
CY2019	dei	Entity Address City Or Town EntityAddressCityOrTown	Cambridge
CY2019	dei	Entity Address State Or Province EntityAddressStateOrProvince	MA
CY2019	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	02142
CY2019	dei	City Area Code CityAreaCode	617
CY2019	dei	Local Phone Number LocalPhoneNumber	679-2000
CY2019	dei	Security12b Title Security12bTitle	Common Stock, $0.0005 par value
CY2019	dei	Trading Symbol TradingSymbol	BIIB
CY2019	dei	Security Exchange Name SecurityExchangeName	NASDAQ
CY2019	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2019	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2019	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2019	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2019	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2019	dei	Entity Small Business EntitySmallBusiness	false
CY2019	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2019	dei	Entity Shell Company EntityShellCompany	false
CY2019Q2	dei	Entity Public Float EntityPublicFloat	43010112437	USD
CY2020Q1	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	174064011	shares
CY2017	us-gaap	Revenues Revenues	12273900000	USD
CY2017	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2253600000	USD
CY2019	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2374700000	USD
CY2019	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	1955400000	USD
CY2018	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	1816300000	USD
CY2017	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	1630000000.0	USD
CY2019	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2280600000	USD
CY2018	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2597200000	USD
CY2019	us-gaap	Revenues Revenues	14377900000	USD
CY2018	us-gaap	Revenues Revenues	13452900000	USD
CY2018	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2106300000	USD
CY2017	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	1933900000	USD
CY2019	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	489900000	USD
CY2018	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	747300000	USD
CY2017	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	814700000	USD
CY2019	biib	Collaboration Profit Sharing CollaborationProfitSharing	241600000	USD
CY2018	biib	Collaboration Profit Sharing CollaborationProfitSharing	185000000.0	USD
CY2017	biib	Collaboration Profit Sharing CollaborationProfitSharing	112300000	USD
CY2019	us-gaap	Disposal Group Not Discontinued Operation Loss Gain On Write Down DisposalGroupNotDiscontinuedOperationLossGainOnWriteDown	55300000	USD
CY2018	us-gaap	Disposal Group Not Discontinued Operation Loss Gain On Write Down DisposalGroupNotDiscontinuedOperationLossGainOnWriteDown	0	USD
CY2017	us-gaap	Disposal Group Not Discontinued Operation Loss Gain On Write Down DisposalGroupNotDiscontinuedOperationLossGainOnWriteDown	0	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	63700000	USD
CY2018	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	12300000	USD
CY2017	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	-62700000	USD
CY2019	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	0	USD
CY2018	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	112500000	USD
CY2017	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	120000000.0	USD
CY2019	us-gaap	Restructuring Charges RestructuringCharges	1500000	USD
CY2018	us-gaap	Restructuring Charges RestructuringCharges	12000000.0	USD
CY2017	us-gaap	Restructuring Charges RestructuringCharges	900000	USD
CY2019	us-gaap	Costs And Expenses CostsAndExpenses	7335300000	USD
CY2018	us-gaap	Costs And Expenses CostsAndExpenses	7564300000	USD
CY2017	us-gaap	Costs And Expenses CostsAndExpenses	6928100000	USD
CY2019	us-gaap	Operating Income Loss OperatingIncomeLoss	7042600000	USD
CY2018	us-gaap	Operating Income Loss OperatingIncomeLoss	5888600000	USD
CY2017	us-gaap	Operating Income Loss OperatingIncomeLoss	5345800000	USD
CY2019	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	83300000	USD
CY2018	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	11000000.0	USD
CY2017	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-217000000.0	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	7125900000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	5899600000	USD
CY2017	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	5128800000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	1158000000.0	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	1425600000	USD
CY2017	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	2458700000	USD
CY2019	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	-79400000	USD
CY2018	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	0	USD
CY2017	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	0	USD
CY2019	us-gaap	Profit Loss ProfitLoss	5888500000	USD
CY2018	us-gaap	Profit Loss ProfitLoss	4474000000.0	USD
CY2017	us-gaap	Profit Loss ProfitLoss	2670100000	USD
CY2019	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	0	USD
CY2018	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	43300000	USD
CY2017	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	131000000.0	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	5888500000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	4430700000	USD
CY2017	us-gaap	Net Income Loss NetIncomeLoss	2539100000	USD
CY2019	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	31.47
CY2018	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	21.63
CY2017	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	11.94
CY2019	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	31.42
CY2018	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	21.58
CY2017	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	11.92
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	187100000	shares
CY2018	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	204900000	shares
CY2017	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	212600000	shares
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	187400000	shares
CY2018	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	205300000	shares
CY2017	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	213000000.0	shares
CY2019	us-gaap	Net Income Loss NetIncomeLoss	5888500000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	4430700000	USD
CY2017	us-gaap	Net Income Loss NetIncomeLoss	2539100000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	11800000	USD
CY2018	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-10600000	USD
CY2017	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-3500000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	3600000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	-6700000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	-12700000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	8200000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	-3900000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	9200000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax	88100000	USD
CY2018	us-gaap	Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax	97400000	USD
CY2017	us-gaap	Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax	-193800000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci On Derivatives Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax	115000000.0	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci On Derivatives Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax	-41800000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci On Derivatives Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax	-31500000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Net Of Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax	-26900000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Net Of Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax	139200000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Derivatives Qualifying As Hedges Net Of Tax OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax	-162300000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Before Adjustments After Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseBeforeAdjustmentsAfterTax	28600000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Before Adjustments After Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseBeforeAdjustmentsAfterTax	5000000.0	USD
CY2017	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Before Adjustments After Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseBeforeAdjustmentsAfterTax	0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Adjustments After Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAdjustmentsAfterTax	7000000.0	USD
CY2018	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Adjustments After Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAdjustmentsAfterTax	1500000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Adjustments After Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAdjustmentsAfterTax	0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	21600000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	3500000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	1500000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	-5500000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	4100000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Foreign Currency Transaction And Translation Adjustment Net Of Tax OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	103800000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Foreign Currency Transaction And Translation Adjustment Net Of Tax OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	-67800000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Foreign Currency Transaction And Translation Adjustment Net Of Tax OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	158700000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	105200000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	76500000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	1500000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	5993700000	USD
CY2018	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	4507200000	USD
CY2017	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	2540600000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	-400000	USD
CY2018	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	42900000	USD
CY2017	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	131000000.0	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	5993300000	USD
CY2018	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	4550100000	USD
CY2017	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	2671600000	USD
CY2019Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	2913700000	USD
CY2018Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	1224600000	USD
CY2019Q4	us-gaap	Available For Sale Securities Current AvailableForSaleSecuritiesCurrent	1562200000	USD
CY2018Q4	us-gaap	Available For Sale Securities Current AvailableForSaleSecuritiesCurrent	2313400000	USD
CY2019Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	1880500000	USD
CY2018Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	1958500000	USD
CY2019Q4	biib	Due From Unconsolidated Joint Business DueFromUnconsolidatedJointBusiness	590200000	USD
CY2018Q4	biib	Due From Unconsolidated Joint Business DueFromUnconsolidatedJointBusiness	526900000	USD
CY2019Q4	us-gaap	Inventory Net InventoryNet	804200000	USD
CY2018Q4	us-gaap	Inventory Net InventoryNet	929900000	USD
CY2019Q4	us-gaap	Other Assets Current OtherAssetsCurrent	631000000.0	USD
CY2018Q4	us-gaap	Other Assets Current OtherAssetsCurrent	687600000	USD
CY2019Q4	us-gaap	Assets Current AssetsCurrent	8381800000	USD
CY2018Q4	us-gaap	Assets Current AssetsCurrent	7640900000	USD
CY2019Q4	us-gaap	Available For Sale Securities Noncurrent AvailableForSaleSecuritiesNoncurrent	1408100000	USD
CY2018Q4	us-gaap	Available For Sale Securities Noncurrent AvailableForSaleSecuritiesNoncurrent	1375900000	USD
CY2019Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3247300000	USD
CY2018Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3601200000	USD
CY2019Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	427000000.0	USD
CY2018Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	0	USD
CY2019Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	3527400000	USD
CY2018Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	3120000000.0	USD
CY2019Q4	us-gaap	Goodwill Goodwill	5757800000	USD
CY2018Q4	us-gaap	Goodwill Goodwill	5706400000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Liabilities Net Noncurrent DeferredTaxAssetsLiabilitiesNetNoncurrent	3232100000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Liabilities Net Noncurrent DeferredTaxAssetsLiabilitiesNetNoncurrent	2153900000	USD
CY2019Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	1252800000	USD
CY2018Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	1690600000	USD
CY2019Q4	us-gaap	Assets Assets	27234300000	USD
CY2018Q4	us-gaap	Assets Assets	25288900000	USD
CY2019Q4	us-gaap	Debt Current DebtCurrent	1495800000	USD
CY2018Q4	us-gaap	Debt Current DebtCurrent	0	USD
CY2019Q4	us-gaap	Taxes Payable Current TaxesPayableCurrent	71400000	USD
CY2018Q4	us-gaap	Taxes Payable Current TaxesPayableCurrent	63500000	USD
CY2019Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	530800000	USD
CY2018Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	370500000	USD
CY2019Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2765800000	USD
CY2018Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2861200000	USD
CY2019Q4	us-gaap	Liabilities Current LiabilitiesCurrent	4863800000	USD
CY2018Q4	us-gaap	Liabilities Current LiabilitiesCurrent	3295200000	USD
CY2019Q4	us-gaap	Long Term Debt LongTermDebt	4459000000.0	USD
CY2018Q4	us-gaap	Long Term Debt LongTermDebt	5936500000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Noncurrent DeferredTaxLiabilitiesNoncurrent	2810800000	USD
CY2018Q4	us-gaap	Deferred Tax Liabilities Noncurrent DeferredTaxLiabilitiesNoncurrent	1636200000	USD
CY2019Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	412700000	USD
CY2018Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	0	USD
CY2019Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1348900000	USD
CY2018Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1389400000	USD
CY2019Q4	us-gaap	Liabilities Liabilities	13895200000	USD
CY2018Q4	us-gaap	Liabilities Liabilities	12257300000	USD
CY2019Q4	us-gaap	Commitments And Contingencies CommitmentsAndContingencies		USD
CY2018Q4	us-gaap	Commitments And Contingencies CommitmentsAndContingencies		USD
CY2019Q4	us-gaap	Preferred Stock Value PreferredStockValue	0	USD
CY2018Q4	us-gaap	Preferred Stock Value PreferredStockValue	0	USD
CY2019Q4	us-gaap	Common Stock Value CommonStockValue	100000	USD
CY2018Q4	us-gaap	Common Stock Value CommonStockValue	100000	USD
CY2019Q4	us-gaap	Additional Paid In Capital AdditionalPaidInCapital	0	USD
CY2018Q4	us-gaap	Additional Paid In Capital AdditionalPaidInCapital	0	USD
CY2019Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-135200000	USD
CY2018Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-240400000	USD
CY2019Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	16455400000	USD
CY2018Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	16257000000.0	USD
CY2019Q4	us-gaap	Treasury Stock Value TreasuryStockValue	2977100000	USD
CY2018Q4	us-gaap	Treasury Stock Value TreasuryStockValue	2977100000	USD
CY2019Q4	us-gaap	Stockholders Equity StockholdersEquity	13343200000	USD
CY2018Q4	us-gaap	Stockholders Equity StockholdersEquity	13039600000	USD
CY2019Q4	us-gaap	Minority Interest MinorityInterest	-4100000	USD
CY2018Q4	us-gaap	Minority Interest MinorityInterest	-8000000.0	USD
CY2019Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13339100000	USD
CY2018Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13031600000	USD
CY2019Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	27234300000	USD
CY2018Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	25288900000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	5888500000	USD
CY2018	us-gaap	Profit Loss ProfitLoss	4474000000.0	USD
CY2017	us-gaap	Profit Loss ProfitLoss	2670100000	USD
CY2019	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	680600000	USD
CY2018	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	1016600000	USD
CY2017	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	1081000000.0	USD
CY2019	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	0	USD
CY2018	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	112500000	USD
CY2017	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	120000000.0	USD
CY2019	us-gaap	Share Based Compensation ShareBasedCompensation	182300000	USD
CY2018	us-gaap	Share Based Compensation ShareBasedCompensation	157500000	USD
CY2017	us-gaap	Share Based Compensation ShareBasedCompensation	128000000.0	USD
CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	67100000	USD
CY2018	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	108300000	USD
CY2017	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	91700000	USD
CY2017	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	-62700000	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	63700000	USD
CY2018	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	12300000	USD
CY2019	us-gaap	Income Loss From Individually Significant Component Disposed Of Or Held For Sale Excluding Discontinued Operations Attributable To Parent Before Income Tax IncomeLossFromIndividuallySignificantComponentDisposedOfOrHeldForSaleExcludingDiscontinuedOperationsAttributableToParentBeforeIncomeTax	-55300000	USD
CY2018	us-gaap	Income Loss From Individually Significant Component Disposed Of Or Held For Sale Excluding Discontinued Operations Attributable To Parent Before Income Tax IncomeLossFromIndividuallySignificantComponentDisposedOfOrHeldForSaleExcludingDiscontinuedOperationsAttributableToParentBeforeIncomeTax	0	USD
CY2017	us-gaap	Income Loss From Individually Significant Component Disposed Of Or Held For Sale Excluding Discontinued Operations Attributable To Parent Before Income Tax IncomeLossFromIndividuallySignificantComponentDisposedOfOrHeldForSaleExcludingDiscontinuedOperationsAttributableToParentBeforeIncomeTax	0	USD
CY2019	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-69200000	USD
CY2018	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	69100000	USD
CY2017	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-162100000	USD
CY2019	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-68800000	USD
CY2018	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	205200000	USD
CY2017	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	435600000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	63300000	USD
CY2018	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-5700000	USD
CY2017	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	232000000.0	USD
CY2019	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	19200000	USD
CY2018	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	52100000	USD
CY2017	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	94500000	USD
CY2019	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	240200000	USD
CY2018	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	465500000	USD
CY2017	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-227400000	USD
CY2019	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	16100000	USD
CY2018	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	321700000	USD
CY2017	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	1303900000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-43300000	USD
CY2018	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-135400000	USD
CY2017	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-79000000.0	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	7078600000	USD
CY2018	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	6187700000	USD
CY2017	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	4551000000.0	USD
CY2019	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	6007000000.0	USD
CY2018	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	9173700000	USD
CY2017	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	5565900000	USD
CY2019	us-gaap	Payments To Acquire Available For Sale Securities PaymentsToAcquireAvailableForSaleSecurities	5252600000	USD
CY2018	us-gaap	Payments To Acquire Available For Sale Securities PaymentsToAcquireAvailableForSaleSecurities	7694800000	USD
CY2017	us-gaap	Payments To Acquire Available For Sale Securities PaymentsToAcquireAvailableForSaleSecurities	5355200000	USD
CY2019	us-gaap	Payments To Acquire Other Productive Assets PaymentsToAcquireOtherProductiveAssets	300000000.0	USD
CY2018	us-gaap	Payments To Acquire Other Productive Assets PaymentsToAcquireOtherProductiveAssets	1500000000.0	USD
CY2017	us-gaap	Payments To Acquire Other Productive Assets PaymentsToAcquireOtherProductiveAssets	1200000000.0	USD
CY2019	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	744400000	USD
CY2018	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	0	USD
CY2017	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	0	USD
CY2019	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	923700000	USD
CY2018	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	USD
CY2017	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	USD
CY2019	us-gaap	Payments To Acquire In Process Research And Development PaymentsToAcquireInProcessResearchAndDevelopment	0	USD
CY2018	us-gaap	Payments To Acquire In Process Research And Development PaymentsToAcquireInProcessResearchAndDevelopment	112500000	USD
CY2017	us-gaap	Payments To Acquire In Process Research And Development PaymentsToAcquireInProcessResearchAndDevelopment	120000000.0	USD
CY2019	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	514500000	USD
CY2018	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	770600000	USD
CY2017	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	867400000	USD
CY2019	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	155000000.0	USD
CY2018	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	3000000.0	USD
CY2017	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	975400000	USD
CY2019	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	0	USD
CY2018	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	462900000	USD
CY2017	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	0	USD
CY2019	us-gaap	Payments To Acquire Equity Method Investments PaymentsToAcquireEquityMethodInvestments	0	USD
CY2018	us-gaap	Payments To Acquire Equity Method Investments PaymentsToAcquireEquityMethodInvestments	676600000	USD
CY2017	us-gaap	Payments To Acquire Equity Method Investments PaymentsToAcquireEquityMethodInvestments	0	USD
CY2019	us-gaap	Proceeds From Sale Of Available For Sale Securities Equity ProceedsFromSaleOfAvailableForSaleSecuritiesEquity	479300000	USD
CY2018	us-gaap	Proceeds From Sale Of Available For Sale Securities Equity ProceedsFromSaleOfAvailableForSaleSecuritiesEquity	0	USD
CY2017	us-gaap	Proceeds From Sale Of Available For Sale Securities Equity ProceedsFromSaleOfAvailableForSaleSecuritiesEquity	0	USD
CY2019	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-27000000.0	USD
CY2018	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-400000	USD
CY2017	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	11000000.0	USD
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	470500000	USD
CY2018	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-2046300000	USD
CY2017	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-2963100000	USD
CY2019	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	5868300000	USD
CY2018	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	4352600000	USD
CY2017	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	1365400000	USD
CY2019	us-gaap	Payments To Minority Shareholders PaymentsToMinorityShareholders	-4300000	USD
CY2018	us-gaap	Payments To Minority Shareholders PaymentsToMinorityShareholders	36400000	USD
CY2017	us-gaap	Payments To Minority Shareholders PaymentsToMinorityShareholders	134100000	USD
CY2019	us-gaap	Repayments Of Debt RepaymentsOfDebt	0	USD
CY2018	us-gaap	Repayments Of Debt RepaymentsOfDebt	3200000	USD
CY2017	us-gaap	Repayments Of Debt RepaymentsOfDebt	560900000	USD
CY2019	us-gaap	Payments Of Capital Distribution PaymentsOfCapitalDistribution	0	USD
CY2018	us-gaap	Payments Of Capital Distribution PaymentsOfCapitalDistribution	0	USD
CY2017	us-gaap	Payments Of Capital Distribution PaymentsOfCapitalDistribution	302700000	USD
CY2019	us-gaap	Payments To Acquire Productive Assets PaymentsToAcquireProductiveAssets	0	USD
CY2018	us-gaap	Payments To Acquire Productive Assets PaymentsToAcquireProductiveAssets	58200000	USD
CY2017	us-gaap	Payments To Acquire Productive Assets PaymentsToAcquireProductiveAssets	3000000.0	USD
CY2019	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	3600000	USD
CY2018	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-21600000	USD
CY2017	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-13900000	USD
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-5860400000	USD
CY2018	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-4472000000.0	USD
CY2017	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-2380000000.0	USD
CY2019	us-gaap	Cash And Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashAndCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	1688700000	USD
CY2018	us-gaap	Cash And Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashAndCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	-330600000	USD
CY2017	us-gaap	Cash And Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashAndCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	-792100000	USD
CY2019	us-gaap	Effect Of Exchange Rate On Cash And Cash Equivalents EffectOfExchangeRateOnCashAndCashEquivalents	400000	USD
CY2018	us-gaap	Effect Of Exchange Rate On Cash And Cash Equivalents EffectOfExchangeRateOnCashAndCashEquivalents	-18600000	USD
CY2017	us-gaap	Effect Of Exchange Rate On Cash And Cash Equivalents EffectOfExchangeRateOnCashAndCashEquivalents	39400000	USD
CY2018Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	1224600000	USD
CY2017Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	1573800000	USD
CY2016Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	2326500000	USD
CY2019Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	2913700000	USD
CY2018Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	1224600000	USD
CY2017Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	1573800000	USD
CY2018Q4	us-gaap	Stockholders Equity StockholdersEquity	13039600000	USD
CY2018Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13031600000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	5888500000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	5888500000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	105200000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	104800000	USD
CY2019	biib	Noncontrollinginterestincreasedecreaseother Noncontrollinginterestincreasedecreaseother	4300000	USD
CY2019	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	40800000	USD
CY2019	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	40800000	USD
CY2019	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	53800000	USD
CY2019	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	53800000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	191200000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	191200000	USD
CY2019Q4	us-gaap	Stockholders Equity StockholdersEquity	13343200000	USD
CY2019Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13339100000	USD
CY2017Q4	us-gaap	Stockholders Equity StockholdersEquity	12612800000	USD
CY2017Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	12598100000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	4430700000	USD
CY2018	us-gaap	Profit Loss ProfitLoss	4474000000.0	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	76500000	USD
CY2018	us-gaap	Treasury Stock Value Acquired Cost Method TreasuryStockValueAcquiredCostMethod	3000000000.0	USD
CY2018	biib	Noncontrollinginterestincreasedecreaseother Noncontrollinginterestincreasedecreaseother	13800000	USD
CY2018	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	50000000.0	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-76100000	USD
CY2018	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	41200000	USD
CY2018	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	41200000	USD
CY2018	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	43800000	USD
CY2018	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	43800000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	168700000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	168700000	USD
CY2018	us-gaap	Stockholders Equity Other StockholdersEquityOther	106100000	USD
CY2018	us-gaap	Stockholders Equity Other StockholdersEquityOther	106100000	USD
CY2018Q4	us-gaap	Stockholders Equity StockholdersEquity	13039600000	USD
CY2018Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13031600000	USD
CY2016Q4	us-gaap	Stockholders Equity StockholdersEquity	12140100000	USD
CY2016Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	12128600000	USD
CY2017	us-gaap	Net Income Loss NetIncomeLoss	2539100000	USD
CY2017	us-gaap	Profit Loss ProfitLoss	2670100000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	1500000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	1500000	USD
CY2017	biib	Noncontrollinginterestincreasedecreaseother Noncontrollinginterestincreasedecreaseother	15800000	USD
CY2017	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	150000000.0	USD
CY2017	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	40500000	USD
CY2017	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	40500000	USD
CY2017	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	45800000	USD
CY2017	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	45800000	USD
CY2016	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	-138100000	USD
CY2016	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	-138100000	USD
CY2017	us-gaap	Stockholders Equity Note Spinoff Transaction StockholdersEquityNoteSpinoffTransaction	-852800000	USD
CY2017	us-gaap	Stockholders Equity Note Spinoff Transaction StockholdersEquityNoteSpinoffTransaction	-852800000	USD
CY2017	us-gaap	Stockholders Equity Note Spinoff Transaction StockholdersEquityNoteSpinoffTransaction	-852800000	USD
CY2017	biib	Stockholders Equity Tax Benefit Expense StockholdersEquityTaxBenefitExpense	-17500000	USD
CY2017Q4	us-gaap	Stockholders Equity StockholdersEquity	12612800000	USD
CY2017Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	12598100000	USD
CY2019	biib	Interest In Subsidiary InterestInSubsidiary	1
CY2019	us-gaap	Use Of Estimates UseOfEstimates	<div style="line-height:120%;padding-top:6px;text-indent:32px;font-size:10pt;"><span style="font-family:Franklin Gothic Book,sans-serif;font-size:10pt;">The preparation of our consolidated financial statements requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenues and expenses and related disclosure of contingent assets and liabilities. On an ongoing basis we evaluate our estimates, judgments and methodologies. We base our estimates on historical experience and on various other assumptions that we believe are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenues and expenses. Actual results may differ from these estimates.</span></div>
CY2019	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	<div style="line-height:120%;padding-top:6px;text-align:left;text-indent:32px;font-size:10pt;"><span style="font-family:Franklin Gothic Book,sans-serif;font-size:10pt;">Financial instruments that potentially subject us to concentrations of credit risk include cash and cash equivalents, investments, derivatives and accounts receivable. We attempt to minimize the risks related to cash and cash equivalents and investments by investing in a broad and diverse range of financial instruments as previously defined by us. We have established guidelines related to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. Our investment portfolio is maintained in accordance with our investment policy, which defines allowable investments, specifies credit quality standards and limits the credit exposure of any single issuer. We minimize credit risk resulting from derivative instruments by choosing only highly rated financial institutions as counterparties.</span></div><div style="line-height:120%;padding-top:12px;text-align:left;text-indent:32px;font-size:10pt;"><span style="font-family:Franklin Gothic Book,sans-serif;font-size:10pt;">Concentrations of credit risk with respect to receivables, which are typically unsecured, are somewhat mitigated due to the wide variety of customers and markets using our products, as well as their dispersion across many different geographic areas. We monitor the financial performance and creditworthiness of our customers so that we can properly assess and respond to changes in their credit profile. We continue to monitor these conditions and assess their possible impact on our business. </span></div>
CY2019	us-gaap	Advertising Expense AdvertisingExpense	79200000	USD
CY2018	us-gaap	Advertising Expense AdvertisingExpense	90200000	USD
CY2017	us-gaap	Advertising Expense AdvertisingExpense	75200000	USD
CY2019Q3	biib	Inventoryrawmaterialssoldto Fujifilm InventoryrawmaterialssoldtoFUJIFILM	41800000	USD
CY2018Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1051300000	USD
CY2019	biib	Current Provisions To Product Revenue Reserves Relating To Sales In Current Year CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear	3698600000	USD
CY2019	biib	Adjustments To Product Revenue Reserves Relating To Sales In Prior Years AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears	48300000	USD
CY2019	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Current Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear	2778600000	USD
CY2019	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear	724100000	USD
CY2019Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1198900000	USD
CY2017Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	761600000	USD
CY2018	biib	Current Provisions To Product Revenue Reserves Relating To Sales In Current Year CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear	3389100000	USD
CY2018	biib	Adjustments To Product Revenue Reserves Relating To Sales In Prior Years AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears	12100000	USD
CY2018	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Current Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear	2440400000	USD
CY2018	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear	646900000	USD
CY2018Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1051300000	USD
CY2016Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	605500000	USD
CY2017	biib	Current Provisions To Product Revenue Reserves Relating To Sales In Current Year CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear	2917300000	USD
CY2017	biib	Adjustments To Product Revenue Reserves Relating To Sales In Prior Years AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears	-6000000.0	USD
CY2017	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Current Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear	2232800000	USD
CY2017	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear	534400000	USD
CY2017Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	761600000	USD
CY2019Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1198900000	USD
CY2018Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1051300000	USD
CY2019	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.16
CY2018	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.15
CY2017	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.13
CY2019Q4	us-gaap	Inventory Raw Materials Net Of Reserves InventoryRawMaterialsNetOfReserves	169700000	USD
CY2018Q4	us-gaap	Inventory Raw Materials Net Of Reserves InventoryRawMaterialsNetOfReserves	196300000	USD
CY2019Q4	us-gaap	Inventory Work In Process Net Of Reserves InventoryWorkInProcessNetOfReserves	460000000.0	USD
CY2018Q4	us-gaap	Inventory Work In Process Net Of Reserves InventoryWorkInProcessNetOfReserves	606700000	USD
CY2019Q4	us-gaap	Inventory Finished Goods Net Of Reserves InventoryFinishedGoodsNetOfReserves	174500000	USD
CY2018Q4	us-gaap	Inventory Finished Goods Net Of Reserves InventoryFinishedGoodsNetOfReserves	133500000	USD
CY2019Q4	biib	Inventorynetcurrentandnoncurrent Inventorynetcurrentandnoncurrent	804200000	USD
CY2018Q4	biib	Inventorynetcurrentandnoncurrent Inventorynetcurrentandnoncurrent	936500000	USD
CY2019Q4	us-gaap	Inventory Net InventoryNet	804200000	USD
CY2018Q4	us-gaap	Inventory Net InventoryNet	929900000	USD
CY2019Q4	us-gaap	Inventory Noncurrent InventoryNoncurrent	0	USD
CY2018Q4	us-gaap	Inventory Noncurrent InventoryNoncurrent	6600000	USD
CY2019Q4	biib	Inventorynetcurrentandnoncurrent Inventorynetcurrentandnoncurrent	804200000	USD
CY2018Q4	biib	Inventorynetcurrentandnoncurrent Inventorynetcurrentandnoncurrent	936500000	USD
CY2019	us-gaap	Inventory Write Down InventoryWriteDown	52200000	USD
CY2018	us-gaap	Inventory Write Down InventoryWriteDown	41900000	USD
CY2017	us-gaap	Inventory Write Down InventoryWriteDown	76900000	USD
CY2019Q3	biib	Inventoryrawmaterialssoldto Fujifilm InventoryrawmaterialssoldtoFUJIFILM	41800000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	7379300000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	4881400000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	2497900000	USD
CY2018Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	7187300000	USD
CY2018Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	4607300000	USD
CY2018Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	2580000000.0	USD
CY2019Q4	us-gaap	Intangible Assets Gross Excluding Goodwill IntangibleAssetsGrossExcludingGoodwill	8408800000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	4881400000	USD
CY2019Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	3527400000	USD
CY2018Q4	us-gaap	Intangible Assets Gross Excluding Goodwill IntangibleAssetsGrossExcludingGoodwill	7727300000	USD
CY2018Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	4607300000	USD
CY2018Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	3120000000.0	USD
CY2019	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	489900000	USD
CY2018	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	747300000	USD
CY2017	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	814700000	USD
CY2019	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	274000000.0	USD
CY2018	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	381200000	USD
CY2017	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	455300000	USD
CY2017Q1	us-gaap	Litigation Settlement Amount Awarded To Other Party LitigationSettlementAmountAwardedToOtherParty	1250000000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Next Twelve Months FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths	260000000.0	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Two FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo	220000000.0	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Three FiniteLivedIntangibleAssetsAmortizationExpenseYearThree	225000000.0	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Four FiniteLivedIntangibleAssetsAmortizationExpenseYearFour	230000000.0	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Five FiniteLivedIntangibleAssetsAmortizationExpenseYearFive	220000000.0	USD
CY2018Q4	us-gaap	Goodwill Goodwill	5706400000	USD
CY2017Q4	us-gaap	Goodwill Goodwill	4632500000	USD
CY2019	us-gaap	Goodwill Acquired During Period GoodwillAcquiredDuringPeriod	117500000	USD
CY2018	us-gaap	Goodwill Acquired During Period GoodwillAcquiredDuringPeriod	1080100000	USD
CY2019	us-gaap	Goodwill Written Off Related To Sale Of Business Unit GoodwillWrittenOffRelatedToSaleOfBusinessUnit	69500000	USD
CY2018	us-gaap	Goodwill Written Off Related To Sale Of Business Unit GoodwillWrittenOffRelatedToSaleOfBusinessUnit	0	USD
CY2019	us-gaap	Goodwill Other Increase Decrease GoodwillOtherIncreaseDecrease	3400000	USD
CY2018	us-gaap	Goodwill Other Increase Decrease GoodwillOtherIncreaseDecrease	-6200000	USD
CY2019Q4	us-gaap	Goodwill Goodwill	5757800000	USD
CY2018Q4	us-gaap	Goodwill Goodwill	5706400000	USD
CY2019Q4	us-gaap	Goodwill Impaired Accumulated Impairment Loss GoodwillImpairedAccumulatedImpairmentLoss	0	USD
CY2019	us-gaap	Servicing Asset At Fair Value Description Of Changes In Valuation Inputs ServicingAssetAtFairValueDescriptionOfChangesInValuationInputs	no
CY2019Q4	us-gaap	Notes Payable Fair Value Disclosure NotesPayableFairValueDisclosure	6553600000	USD
CY2018Q4	us-gaap	Notes Payable Fair Value Disclosure NotesPayableFairValueDisclosure	6037600000	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	63700000	USD
CY2018	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	12300000	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements	0	USD
CY2018	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements	101500000	USD
CY2019	us-gaap	Debt Instrument Maturity Date Description DebtInstrumentMaturityDateDescription	less than 90 days
CY2019Q4	us-gaap	Cash Equivalents At Carrying Value CashEquivalentsAtCarryingValue	2541100000	USD
CY2018Q4	us-gaap	Cash Equivalents At Carrying Value CashEquivalentsAtCarryingValue	705500000	USD
CY2019Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2965000000.0	USD
CY2019Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	6000000.0	USD
CY2019Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	700000	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2970300000	USD
CY2018Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	3694000000.0	USD
CY2018Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	1300000	USD
CY2018Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	6000000.0	USD
CY2018Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	3689300000	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Fair Value AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue	1562200000	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost	1560800000	USD
CY2018Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Fair Value AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue	2313400000	USD
CY2018Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost	2314600000	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	1234500000	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost	1230400000	USD
CY2018Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	1232700000	USD
CY2018Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost	1235900000	USD
CY2019Q4	biib	Available For Sale Securities Debt Maturities After Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsFairValue	173600000	USD
CY2019Q4	biib	Available For Sale Securities Debt Maturities After Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsAmortizedCost	173800000	USD
CY2018Q4	biib	Available For Sale Securities Debt Maturities After Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsFairValue	143200000	USD
CY2018Q4	biib	Available For Sale Securities Debt Maturities After Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsAmortizedCost	143500000	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2970300000	USD
CY2019Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2965000000.0	USD
CY2018Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	3689300000	USD
CY2018Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	3694000000.0	USD
CY2019	biib	Average Maturity Of Marketable Securities AverageMaturityOfMarketableSecurities	P14M
CY2018	biib	Average Maturity Of Marketable Securities AverageMaturityOfMarketableSecurities	P12M
CY2017	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	4500000	USD
CY2019	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	6007000000.0	USD
CY2018	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	9173700000	USD
CY2017	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	5565900000	USD
CY2019	us-gaap	Available For Sale Securities Gross Realized Gains AvailableForSaleSecuritiesGrossRealizedGains	6000000.0	USD
CY2018	us-gaap	Available For Sale Securities Gross Realized Gains AvailableForSaleSecuritiesGrossRealizedGains	3200000	USD
CY2017	us-gaap	Available For Sale Securities Gross Realized Gains AvailableForSaleSecuritiesGrossRealizedGains	3000000.0	USD
CY2019	us-gaap	Available For Sale Securities Gross Realized Losses AvailableForSaleSecuritiesGrossRealizedLosses	1500000	USD
CY2018	us-gaap	Available For Sale Securities Gross Realized Losses AvailableForSaleSecuritiesGrossRealizedLosses	11700000	USD
CY2017	us-gaap	Available For Sale Securities Gross Realized Losses AvailableForSaleSecuritiesGrossRealizedLosses	22400000	USD
CY2019	us-gaap	Unrealized Gain Loss On Foreign Currency Derivatives Net Before Tax UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax	500000	USD
CY2018	us-gaap	Unrealized Gain Loss On Foreign Currency Derivatives Net Before Tax UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax	-27300000	USD
CY2017	us-gaap	Unrealized Gain Loss On Foreign Currency Derivatives Net Before Tax UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax	113000000.0	USD
CY2019	us-gaap	Unrealized Gain Loss On Foreign Currency Derivatives Net Before Tax UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax	500000	USD
CY2019	us-gaap	Derivative Remaining Maturity1 DerivativeRemainingMaturity1	P15M
CY2019	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	5900000	USD
CY2018	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	2000000.0	USD
CY2019Q4	us-gaap	Land Land	118100000	USD
CY2018Q4	us-gaap	Land Land	144500000	USD
CY2019Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	835000000.0	USD
CY2018Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	1282800000	USD
CY2019Q4	us-gaap	Leasehold Improvements Gross LeaseholdImprovementsGross	99500000	USD
CY2018Q4	us-gaap	Leasehold Improvements Gross LeaseholdImprovementsGross	94400000	USD
CY2019Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	844500000	USD
CY2018Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	1258100000	USD
CY2019Q4	biib	Computer Software And Hardware ComputerSoftwareAndHardware	798400000	USD
CY2018Q4	biib	Computer Software And Hardware ComputerSoftwareAndHardware	798700000	USD
CY2019Q4	us-gaap	Furniture And Fixtures Gross FurnitureAndFixturesGross	58300000	USD
CY2018Q4	us-gaap	Furniture And Fixtures Gross FurnitureAndFixturesGross	61600000	USD
CY2019Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	2084400000	USD
CY2018Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	1758500000	USD
CY2019Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	4838200000	USD
CY2018Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	5398600000	USD
CY2019Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	1590900000	USD
CY2018Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	1797400000	USD
CY2019Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3247300000	USD
CY2018Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3601200000	USD
CY2019	us-gaap	Depreciation Depreciation	190600000	USD
CY2018	us-gaap	Depreciation Depreciation	269400000	USD
CY2017	us-gaap	Depreciation Depreciation	266300000	USD
CY2019	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	68800000	USD
CY2018	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	54000000.0	USD
CY2017	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	30700000	USD
CY2019Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	486300000	USD
CY2019	us-gaap	Lease Cost LeaseCost	86700000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Remainder Of Fiscal Year LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear	87600000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	81500000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	75700000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	72500000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	69000000.0	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due After Year Five LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive	158300000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	544600000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	58300000	USD
CY2019Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	486300000	USD
CY2018Q4	us-gaap	Operating Leases Future Minimum Payments Due Current OperatingLeasesFutureMinimumPaymentsDueCurrent	87000000.0	USD
CY2018Q4	us-gaap	Operating Leases Future Minimum Payments Due In Two Years OperatingLeasesFutureMinimumPaymentsDueInTwoYears	80700000	USD
CY2018Q4	us-gaap	Operating Leases Future Minimum Payments Due In Three Years OperatingLeasesFutureMinimumPaymentsDueInThreeYears	75900000	USD
CY2018Q4	us-gaap	Operating Leases Future Minimum Payments Due In Four Years OperatingLeasesFutureMinimumPaymentsDueInFourYears	71700000	USD
CY2018Q4	us-gaap	Operating Leases Future Minimum Payments Due In Five Years OperatingLeasesFutureMinimumPaymentsDueInFiveYears	71000000.0	USD
CY2018Q4	us-gaap	Operating Leases Future Minimum Payments Due Thereafter OperatingLeasesFutureMinimumPaymentsDueThereafter	215300000	USD
CY2018Q4	us-gaap	Operating Leases Future Minimum Payments Due OperatingLeasesFutureMinimumPaymentsDue	601600000	USD
CY2018Q4	us-gaap	Future Minimum Sublease Rentals Sale Leaseback Transactions Within One Year FutureMinimumSubleaseRentalsSaleLeasebackTransactionsWithinOneYear	26800000	USD
CY2018Q4	us-gaap	Future Minimum Sublease Rentals Sale Leaseback Transactions Within Two Years FutureMinimumSubleaseRentalsSaleLeasebackTransactionsWithinTwoYears	25600000	USD
CY2018Q4	us-gaap	Future Minimum Sublease Rentals Sale Leaseback Transactions Within Three Years FutureMinimumSubleaseRentalsSaleLeasebackTransactionsWithinThreeYears	23700000	USD
CY2018Q4	us-gaap	Future Minimum Sublease Rentals Sale Leaseback Transactions Within Four Years FutureMinimumSubleaseRentalsSaleLeasebackTransactionsWithinFourYears	24000000.0	USD
CY2018Q4	us-gaap	Future Minimum Sublease Rentals Sale Leaseback Transactions Within Five Years FutureMinimumSubleaseRentalsSaleLeasebackTransactionsWithinFiveYears	24300000	USD
CY2018Q4	us-gaap	Future Minimum Sublease Rentals Sale Leaseback Transactions Thereafter FutureMinimumSubleaseRentalsSaleLeasebackTransactionsThereafter	58400000	USD
CY2018Q4	us-gaap	Future Minimum Sublease Rentals Sale Leaseback Transactions FutureMinimumSubleaseRentalsSaleLeasebackTransactions	182800000	USD
CY2018Q4	biib	Net Minimum Lease Payments With In One Year NetMinimumLeasePaymentsWithInOneYear	60200000	USD
CY2018Q4	biib	Net Minimum Lease Payments With In Two Year NetMinimumLeasePaymentsWithInTwoYear	55100000	USD
CY2018Q4	biib	Net Minimum Lease Payments With In Three Year NetMinimumLeasePaymentsWithInThreeYear	52200000	USD
CY2018Q4	biib	Net Minimum Lease Payments With In Four Year NetMinimumLeasePaymentsWithInFourYear	47700000	USD
CY2018Q4	biib	Net Minimum Lease Payments With In Five Year NetMinimumLeasePaymentsWithInFiveYear	46700000	USD
CY2018Q4	biib	Net Minimum Lease Payments Thereafter NetMinimumLeasePaymentsThereafter	156900000	USD
CY2018Q4	biib	Net Minimum Lease Payments NetMinimumLeasePayments	418800000	USD
CY2019Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P7Y25D
CY2019Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.032
CY2019	us-gaap	Operating Lease Payments OperatingLeasePayments	93800000	USD
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	35900000	USD
CY2019Q4	us-gaap	Debt Current DebtCurrent	1495800000	USD
CY2018Q4	us-gaap	Debt Current DebtCurrent	0	USD
CY2019Q4	us-gaap	Long Term Debt LongTermDebt	4459000000.0	USD
CY2018Q4	us-gaap	Long Term Debt LongTermDebt	5936500000	USD
CY2015	us-gaap	Payments Of Financing Costs PaymentsOfFinancingCosts	47500000	USD
CY2019	us-gaap	Debt Instrument Redemption Price Percentage DebtInstrumentRedemptionPricePercentage	1
CY2019	biib	Debt Instrument Make Whole Provision Redemption Price Percentage DebtInstrumentMakeWholeProvisionRedemptionPricePercentage	1.01
CY2015Q4	us-gaap	Line Of Credit Facility Maximum Borrowing Capacity LineOfCreditFacilityMaximumBorrowingCapacity	1000000000.0	USD
CY2015	us-gaap	Line Of Credit Facility Description LineOfCreditFacilityDescription	five
CY2019Q4	us-gaap	Line Of Credit Facility Fair Value Of Amount Outstanding LineOfCreditFacilityFairValueOfAmountOutstanding	0	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	1500000000.0	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	0	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	1000000000.0	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	0	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive	0	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal After Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive	3500000000.0	USD
CY2019Q4	us-gaap	Long Term Notes Payable LongTermNotesPayable	6000000000.0	USD
CY2019Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-135200000	USD
CY2017Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-318400000	USD
CY2018Q1	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-316900000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	29500000	USD
CY2018	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	47000000.0	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	76500000	USD
CY2019Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	8000000.0	shares
CY2019Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000.0	shares
CY2019Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	198000000.0	shares
CY2019Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	174200000	shares
CY2018Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000.0	shares
CY2018Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	221000000.0	shares
CY2018Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	197200000	shares
CY2017Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000.0	shares
CY2017Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	235300000	shares
CY2017Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	211500000	shares
CY2018Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-240400000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	230800000	USD
CY2019	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	-125600000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	105200000	USD
CY2018Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-240400000	USD
CY2016Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-319900000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	-42700000	USD
CY2017	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	44200000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	1500000	USD
CY2017Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-318400000	USD
CY2019	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	1143900000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	5888500000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	4430700000	USD
CY2017	us-gaap	Net Income Loss NetIncomeLoss	2539100000	USD
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	187100000	shares
CY2018	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	204900000	shares
CY2017	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	212600000	shares
CY2019	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	300000	shares
CY2018	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	400000	shares
CY2017	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	400000	shares
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	187400000	shares
CY2018	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	205300000	shares
CY2017	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	213000000.0	shares
CY2019	us-gaap	Weighted Average Number Of Shares Common Stock Subject To Repurchase Or Cancellation WeightedAverageNumberOfSharesCommonStockSubjectToRepurchaseOrCancellation	23600000	shares
CY2018	us-gaap	Weighted Average Number Of Shares Common Stock Subject To Repurchase Or Cancellation WeightedAverageNumberOfSharesCommonStockSubjectToRepurchaseOrCancellation	14800000	shares
CY2017	us-gaap	Weighted Average Number Of Shares Common Stock Subject To Repurchase Or Cancellation WeightedAverageNumberOfSharesCommonStockSubjectToRepurchaseOrCancellation	3700000	shares
CY2019	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	225400000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	181600000	USD
CY2017	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	169700000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8900000	USD
CY2018	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	11500000	USD
CY2017	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	9600000	USD
CY2019	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	216500000	USD
CY2018	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	170100000	USD
CY2017	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	160100000	USD
CY2019Q4	us-gaap	Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized	210400000	USD
CY2019	biib	Weighted Average Period To Recognize Cost Of Nonvested Awards WeightedAveragePeriodToRecognizeCostOfNonvestedAwards	P1Y10M24D
CY2019	biib	Number Of Share Based Compensation Plans Pursuant To Which Awards Are Currently Being Made NumberOfShareBasedCompensationPlansPursuantToWhichAwardsAreCurrentlyBeingMade	3	Plan
CY2017	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	9600000	USD
CY2019	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	216500000	USD
CY2018	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	170100000	USD
CY2017	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	160100000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	35700000	USD
CY2018	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	27500000	USD
CY2017	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	42800000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	225400000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	181600000	USD
CY2017	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	169700000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8900000	USD
CY2018	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	11500000	USD
CY2018Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	27000	shares
CY2018Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	53.82
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod	0	shares
CY2019	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	0
CY2019	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	15000	shares
CY2019	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	50.03
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod	0	shares
CY2019	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Forfeitures In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice	0
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	12000	shares
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	58.46
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	4200000	USD
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	4000000.0	USD
CY2017	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	3400000	USD
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	2900000	USD
CY2019	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P0Y2M12D
CY2019	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	2500000	USD
CY2018	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	2200000	USD
CY2017	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	3400000	USD
CY2019	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	400000	USD
CY2018	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	800000	USD
CY2017	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	700000	USD
CY2019	us-gaap	Stock Issued During Period Shares Employee Stock Purchase Plans StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans	6200000	shares
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	4725300000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	3877000000.0	USD
CY2017	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	3540400000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	2400600000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	2022600000	USD
CY2017	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	1588400000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	7125900000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	5899600000	USD
CY2017	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	5128800000	USD
CY2019	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	947400000	USD
CY2018	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	1131800000	USD
CY2017	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	2201400000	USD
CY2019	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	59100000	USD
CY2018	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	45500000	USD
CY2018	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-62000000.0	USD
CY2017	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	57000000.0	USD
CY2019	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	84400000	USD
CY2018	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	140000000.0	USD
CY2017	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	108600000	USD
CY2019	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	1090900000	USD
CY2018	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	1317300000	USD
CY2017	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	2367000000.0	USD
CY2017	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	241000000.0	USD
CY2019	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-2300000	USD
CY2018	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-7400000	USD
CY2017	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	9900000	USD
CY2019	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-1074500000	USD
CY2018	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	177700000	USD
CY2017	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-159200000	USD
CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	67100000	USD
CY2018	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	108300000	USD
CY2017	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	91700000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	1158000000.0	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	1425600000	USD
CY2017	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	2458700000	USD
CY2017	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.35
CY2019	us-gaap	Income Taxes Paid IncomeTaxesPaid	87000000.0	USD
CY2019Q4	biib	Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries	1500000000	USD
CY2017Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Impairment Losses DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsImpairmentLosses	48800000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	106600000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	102800000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Inventory DeferredTaxAssetsInventory	162000000.0	USD
CY2018Q4	us-gaap	Deferred Tax Assets Inventory DeferredTaxAssetsInventory	163900000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Goodwill And Intangible Assets DeferredTaxAssetsGoodwillAndIntangibleAssets	3380000000.0	USD
CY2018Q4	us-gaap	Deferred Tax Assets Goodwill And Intangible Assets DeferredTaxAssetsGoodwillAndIntangibleAssets	2298600000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	130400000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	213100000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	23800000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	25800000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	103700000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	38900000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	1100000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	20000000.0	USD
CY2019Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	3905400000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2823100000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets	350300000	USD
CY2018Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets	232800000	USD
CY2019Q4	biib	Deferredtaxliabilities Gilt Itaxcalculation DeferredtaxliabilitiesGILTItaxcalculation	-1381600000	USD
CY2018Q4	biib	Deferredtaxliabilities Gilt Itaxcalculation DeferredtaxliabilitiesGILTItaxcalculation	-544600000	USD
CY2019Q4	biib	Deferred Tax Liabilities Tax Credit Carryforwards DeferredTaxLiabilitiesTaxCreditCarryforwards	-1617200000	USD
CY2018Q4	biib	Deferred Tax Liabilities Tax Credit Carryforwards DeferredTaxLiabilitiesTaxCreditCarryforwards	-1425700000	USD
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Other EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther	0.002
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Other EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther	0.003
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Other EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther	0.007
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Change In Enacted Tax Rate EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate	0
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Change In Enacted Tax Rate EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate	0.021
CY2019Q4	biib	Deferred Tax Liabilities Depreciation Amortization Other DeferredTaxLiabilitiesDepreciationAmortizationOther	135000000.0	USD
CY2018Q4	biib	Deferred Tax Liabilities Depreciation Amortization Other DeferredTaxLiabilitiesDepreciationAmortizationOther	102300000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	3484100000	USD
CY2018Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	2305400000	USD
CY2019Q4	biib	Totaldeferredchargesandprepaidtaxes Totaldeferredchargesandprepaidtaxes	243800000	USD
CY2018Q4	biib	Totaldeferredchargesandprepaidtaxes Totaldeferredchargesandprepaidtaxes	239200000	USD
CY2019	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2018	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2017	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.350
CY2019	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.008
CY2018	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.006
CY2017	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.008
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.045
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.019
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.111
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.011
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.009
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.008
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	0.004
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	0.012
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	0.014
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Assets EffectiveIncomeTaxRateReconciliationDispositionOfAssets	0.010
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Assets EffectiveIncomeTaxRateReconciliationDispositionOfAssets	0
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Assets EffectiveIncomeTaxRateReconciliationDispositionOfAssets	0
CY2019	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	-0.021
CY2018	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	0
CY2017	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	0
CY2019	biib	Effective Tax Rate Reconciliation Gilt Itax EffectiveTaxRateReconciliationGILTItax	0.015
CY2018	biib	Effective Tax Rate Reconciliation Gilt Itax EffectiveTaxRateReconciliationGILTItax	0.016
CY2017	biib	Effective Tax Rate Reconciliation Gilt Itax EffectiveTaxRateReconciliationGILTItax	0
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Change In Enacted Tax Rate EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate	0.229
CY2019	biib	Effective Income Tax Rate Reconciliation Changein Foreign Tax Rate Percent EffectiveIncomeTaxRateReconciliationChangeinForeignTaxRatePercent	-0.008
CY2018	biib	Effective Income Tax Rate Reconciliation Changein Foreign Tax Rate Percent EffectiveIncomeTaxRateReconciliationChangeinForeignTaxRatePercent	0
CY2017	biib	Effective Income Tax Rate Reconciliation Changein Foreign Tax Rate Percent EffectiveIncomeTaxRateReconciliationChangeinForeignTaxRatePercent	0
CY2019	biib	Effective Income Tax Rate Reconciliation Manufacturing Deduction EffectiveIncomeTaxRateReconciliationManufacturingDeduction	0
CY2018	biib	Effective Income Tax Rate Reconciliation Manufacturing Deduction EffectiveIncomeTaxRateReconciliationManufacturingDeduction	0
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Impairment Losses EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses	0.009
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	-0.001
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0.002
CY2017	biib	Effective Income Tax Rate Reconciliation Manufacturing Deduction EffectiveIncomeTaxRateReconciliationManufacturingDeduction	-0.019
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Impairment Losses EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses	0
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Impairment Losses EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses	0
CY2017	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0
CY2019	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.163
CY2018	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.242
CY2017	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.479
CY2017Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	66800000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	122700000	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	109100000	USD
CY2017Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	64300000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	4700000	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	114200000	USD
CY2017Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	66800000	USD
CY2016Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	32400000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	5300000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	500000	USD
CY2017	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	5700000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	17200000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	58700000	USD
CY2017	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	7300000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	10300000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	13600000	USD
CY2017	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	21800000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	100000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	2900000	USD
CY2017	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	1400000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	3600000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	4700000	USD
CY2017	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	44600000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	129900000	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	114200000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	2200000	USD
CY2017	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	4800000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	20000000.0	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	13800000	USD
CY2019Q4	us-gaap	Decrease In Unrecognized Tax Benefits Is Reasonably Possible DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible	75000000.0	USD
CY2019	us-gaap	Interest Paid Net InterestPaidNet	244200000	USD
CY2018	us-gaap	Interest Paid Net InterestPaidNet	243200000	USD
CY2017	us-gaap	Interest Paid Net InterestPaidNet	281700000	USD
CY2019	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	1064500000	USD
CY2018	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	1007100000	USD
CY2017	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	1066400000	USD
CY2019	us-gaap	Investment Income Interest InvestmentIncomeInterest	120000000.0	USD
CY2018	us-gaap	Investment Income Interest InvestmentIncomeInterest	112500000	USD
CY2017	us-gaap	Investment Income Interest InvestmentIncomeInterest	78500000	USD
CY2019	us-gaap	Interest Expense InterestExpense	187400000	USD
CY2018	us-gaap	Interest Expense InterestExpense	200600000	USD
CY2017	us-gaap	Interest Expense InterestExpense	250800000	USD
CY2019	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	204700000	USD
CY2018	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	119500000	USD
CY2017	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	-36300000	USD
CY2019	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	-7000000.0	USD
CY2018	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	-9900000	USD
CY2017	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	6300000	USD
CY2019	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	-47000000.0	USD
CY2018	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	-10500000	USD
CY2017	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	-14700000	USD
CY2019	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	83300000	USD
CY2018	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	11000000.0	USD
CY2017	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-217000000.0	USD
CY2019Q4	us-gaap	Business Combination Contingent Consideration Liability Current BusinessCombinationContingentConsiderationLiabilityCurrent	148400000	USD
CY2018Q4	us-gaap	Business Combination Contingent Consideration Liability Current BusinessCombinationContingentConsiderationLiabilityCurrent	444800000	USD
CY2019Q4	us-gaap	Construction Payable Current And Noncurrent ConstructionPayableCurrentAndNoncurrent	78000000.0	USD
CY2019	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	200100000	USD
CY2018	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	127900000	USD
CY2017	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	-19800000	USD
CY2019	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	-50000000.0	USD
CY2018	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	600000	USD
CY2017	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	0	USD
CY2019	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	150100000	USD
CY2018	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	128500000	USD
CY2017	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	-19800000	USD
CY2019Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	309100000	USD
CY2018Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	320900000	USD
CY2019Q4	biib	Collaborationexpensesaccrual Collaborationexpensesaccrual	281600000	USD
CY2018Q4	biib	Collaborationexpensesaccrual Collaborationexpensesaccrual	261600000	USD
CY2019Q4	us-gaap	Accrued Royalties Current AccruedRoyaltiesCurrent	220900000	USD
CY2018Q4	us-gaap	Accrued Royalties Current AccruedRoyaltiesCurrent	224700000	USD
CY2018Q4	us-gaap	Construction Payable Current And Noncurrent ConstructionPayableCurrentAndNoncurrent	125200000	USD
CY2019Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	726700000	USD
CY2018Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	609300000	USD
CY2019Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2765800000	USD
CY2018Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2861200000	USD
CY2019Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1348900000	USD
CY2018Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1389400000	USD
CY2019Q4	biib	Potential Future Milestone Payments Commitment To Third Party Approximately PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately	6800000000	USD
CY2019Q4	us-gaap	Accrued Income Taxes Noncurrent AccruedIncomeTaxesNoncurrent	803300000	USD
CY2018Q4	us-gaap	Accrued Income Taxes Noncurrent AccruedIncomeTaxesNoncurrent	791400000	USD
CY2019Q4	biib	Investment In Biotechnology Companies That Are Determined To Be Unconsolidated Variable Interest Entities InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities	22700000	USD
CY2018Q4	biib	Investment In Biotechnology Companies That Are Determined To Be Unconsolidated Variable Interest Entities InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities	28700000	USD
CY2019	us-gaap	Loss Contingency Damages Sought Value LossContingencyDamagesSoughtValue	200000000.0	USD
CY2019Q4	us-gaap	Business Combination Contingent Consideration Arrangements Range Of Outcomes Value High BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh	735000000.0	USD
CY2019Q4	us-gaap	Gross Profit GrossProfit	3224200000	USD
CY2019	us-gaap	Gross Profit GrossProfit	12422500000	USD
CY2019Q1	us-gaap	Profit Loss ProfitLoss	1408800000	USD
CY2019Q2	us-gaap	Profit Loss ProfitLoss	1494100000	USD
CY2019Q4	biib	Accrued Expenses On Funding Commitment Related To Clinical Research AccruedExpensesOnFundingCommitmentRelatedToClinicalResearch	24000000.0	USD
CY2019Q4	us-gaap	Other Commitment OtherCommitment	514000000.0	USD
CY2019Q4	us-gaap	Liability For Uncertain Tax Positions Current LiabilityForUncertainTaxPositionsCurrent	136900000	USD
CY2019	us-gaap	Income Taxes Paid IncomeTaxesPaid	87000000.0	USD
CY2019	us-gaap	Number Of Reportable Segments NumberOfReportableSegments	1	segment
CY2019Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3674300000	USD
CY2018Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3601200000	USD
CY2017Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3182400000	USD
CY2019Q1	us-gaap	Revenues Revenues	3489800000	USD
CY2019Q2	us-gaap	Revenues Revenues	3616700000	USD
CY2019Q3	us-gaap	Revenues Revenues	3600100000	USD
CY2019Q4	us-gaap	Revenues Revenues	3671300000	USD
CY2019	us-gaap	Revenues Revenues	14377900000	USD
CY2019Q1	us-gaap	Gross Profit GrossProfit	2887800000	USD
CY2019Q2	us-gaap	Gross Profit GrossProfit	3140400000	USD
CY2019Q3	us-gaap	Gross Profit GrossProfit	3170100000	USD
CY2019Q3	us-gaap	Profit Loss ProfitLoss	1545900000	USD
CY2019Q4	us-gaap	Profit Loss ProfitLoss	1439700000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	5888500000	USD
CY2019Q1	us-gaap	Net Income Loss NetIncomeLoss	1408800000	USD
CY2019Q2	us-gaap	Net Income Loss NetIncomeLoss	1494100000	USD
CY2019Q3	us-gaap	Net Income Loss NetIncomeLoss	1545900000	USD
CY2019Q4	us-gaap	Net Income Loss NetIncomeLoss	1439700000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	5888500000	USD
CY2019Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	7.17
CY2019Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	7.85
CY2018Q4	us-gaap	Gross Profit GrossProfit	3037800000	USD
CY2018	us-gaap	Gross Profit GrossProfit	11636600000	USD
CY2018Q1	us-gaap	Profit Loss ProfitLoss	1171200000	USD
CY2018Q2	us-gaap	Profit Loss ProfitLoss	915000000.0	USD
CY2018Q3	us-gaap	Profit Loss ProfitLoss	1442900000	USD
CY2018Q4	us-gaap	Profit Loss ProfitLoss	944900000	USD
CY2018	us-gaap	Profit Loss ProfitLoss	4474000000.0	USD
CY2018Q1	us-gaap	Net Income Loss NetIncomeLoss	1172900000	USD
CY2019Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	8.40
CY2019Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	8.10
CY2019	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	31.47
CY2019Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	7.15
CY2019Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	7.85
CY2019Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	8.39
CY2019Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	8.08
CY2019	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	31.42
CY2019Q1	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	196600000	shares
CY2019Q2	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	190300000	shares
CY2019Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	184000000.0	shares
CY2019Q4	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	177800000	shares
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	187100000	shares
CY2019Q1	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	197000000.0	shares
CY2019Q2	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	190400000	shares
CY2019Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	184200000	shares
CY2019Q4	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	178200000	shares
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	187400000	shares
CY2018Q1	us-gaap	Revenues Revenues	3131100000	USD
CY2018Q2	us-gaap	Revenues Revenues	3356500000	USD
CY2018Q3	us-gaap	Revenues Revenues	3439000000.0	USD
CY2018Q4	us-gaap	Revenues Revenues	3526300000	USD
CY2018	us-gaap	Revenues Revenues	13452900000	USD
CY2018Q1	us-gaap	Gross Profit GrossProfit	2685100000	USD
CY2018Q2	us-gaap	Gross Profit GrossProfit	2935500000	USD
CY2018Q3	us-gaap	Gross Profit GrossProfit	2978200000	USD
CY2018Q2	us-gaap	Net Income Loss NetIncomeLoss	866600000	USD
CY2018Q3	us-gaap	Net Income Loss NetIncomeLoss	1444400000	USD
CY2018Q4	us-gaap	Net Income Loss NetIncomeLoss	946800000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	4430700000	USD
CY2018Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	5.55
CY2018Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	4.18
CY2018Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	7.17
CY2018Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	4.74
CY2018	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	21.63
CY2018Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	5.54
CY2018Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	4.18
CY2018Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	7.15
CY2018Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	4.73
CY2018	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	21.58
CY2018Q1	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	211400000	shares
CY2018Q2	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	207100000	shares
CY2018Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	201400000	shares
CY2018Q4	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	199800000	shares
CY2018	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	204900000	shares
CY2018Q1	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	211700000	shares
CY2018Q2	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	207300000	shares
CY2018Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	201900000	shares
CY2018Q4	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	200300000	shares
CY2018	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	205300000	shares
CY2019Q1	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	376100000	USD
CY2019Q2	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	-174200000	USD
CY2019Q3	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	-4600000	USD
CY2019Q4	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	2800000	USD
CY2019Q1	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	-11500000	USD
CY2019Q2	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	20000000.0	USD
CY2019Q3	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	57800000	USD
CY2019Q4	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	-2600000	USD
CY2018Q1	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	-6400000	USD
CY2018Q2	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	5400000	USD
CY2018Q3	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	141200000	USD
CY2018Q4	us-gaap	Unrealized Gain Loss On Marketable Securities Cost Method Investments And Other Investments UnrealizedGainLossOnMarketableSecuritiesCostMethodInvestmentsAndOtherInvestments	-12200000	USD
CY2018Q1	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	5600000	USD
CY2018Q2	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	-1900000	USD
CY2018Q3	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	87900000	USD
CY2018Q4	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	-79300000	USD

Files In Submission

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$BIIB
BIOGEN INC. Stock $biib

2019 Q4 Form 10-K Financial Statement

#000087504520000005 Filed on February 06, 2020

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$BIIB BIOGEN INC. Stock $biib

2019 Q4 Form 10-K Financial Statement

#000087504520000005 Filed on February 06, 2020

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$BIIB
BIOGEN INC. Stock $biib