$BIIB
BIOGEN INC. Stock $biib

$biib Overview Financials Valuation SEC Filings Company Website

2021 Q4 Form 10-K Financial Statement

#000087504522000007 Filed on February 03, 2022

Income Statement

Concept	2021 Q4	2021	2020 Q4
Revenue	$2.734B	$10.98B	$2.853B
YoY Change	-4.16%	-18.32%	-22.3%
Cost Of Revenue	$660.1M	$2.110B	$490.6M
YoY Change	34.55%	16.87%	9.73%
Gross Profit	$2.074B	$8.872B	$2.362B
YoY Change	-12.2%	-23.78%	-26.74%
Gross Profit Margin	75.85%	80.79%	82.8%
Selling, General & Admin	$787.9M	$2.674B	$806.2M
YoY Change	-2.27%	6.78%	21.97%
% of Gross Profit	37.99%	30.14%	34.13%
Research & Development	$699.5M	$2.501B	$1.726B
YoY Change	-59.48%	-37.33%	150.41%
% of Gross Profit	33.73%	28.19%	73.08%
Depreciation & Amortization	$134.7M	$487.7M	$90.40M
YoY Change	49.0%	6.67%	-20.0%
% of Gross Profit	6.5%	5.5%	3.83%
Operating Expenses	$1.554B	$5.176B	$2.528B
YoY Change	-38.54%	-20.32%	77.19%
Operating Profit	$519.8M	$2.841B	-$166.4M
YoY Change	-412.38%	-37.57%	-109.26%
Interest Expense	-$113.2M	$253.6M	$614.6M
YoY Change	-118.42%	13.98%	-945.39%
% of Operating Profit	-21.78%	8.93%
Other Income/Expense, Net	-$1.600M	-$1.096B	$2.500M
YoY Change	-164.0%	-320.25%	-106.87%
Pretax Income	$405.0M	$448.3M	$352.9M
YoY Change	14.76%	-86.37%	-79.58%
Income Tax	$443.2M	$52.50M	$13.30M
% Of Pretax Income	109.43%	11.71%	3.77%
Net Earnings	$368.2M	$1.556B	$357.9M
YoY Change	2.88%	-61.1%	-75.14%
Net Earnings / Revenue	13.47%	14.17%	12.55%
Basic Earnings Per Share		$10.44	$2.33
Diluted Earnings Per Share	$2.496M	$10.40	$2.32
COMMON SHARES
Basic Shares Outstanding	146.9M shares	149.1M shares	153.7M shares
Diluted Shares Outstanding		149.6M shares	154.0M shares

Balance Sheet

Concept	2021 Q4	2021	2020 Q4
SHORT-TERM ASSETS
Cash & Short-Term Investments	$3.803B	$3.803B	$2.610B
YoY Change	45.68%	45.69%	-41.69%
Cash & Equivalents	$2.261B	$2.261B	$1.331B
Short-Term Investments	$1.541B	$1.541B	$1.279B
Other Short-Term Assets	$740.8M	$740.8M	$881.1M
YoY Change	-15.92%	-15.91%	39.64%
Inventory	$1.352B	$1.352B	$1.069B
Prepaid Expenses
Receivables	$1.549B	$1.962B	$1.914B
Other Receivables	$0.00	$0.00	$0.00
Total Short-Term Assets	$7.857B	$7.857B	$6.887B
YoY Change	14.08%	14.08%	-17.83%
LONG-TERM ASSETS
Property, Plant & Equipment	$3.416B	$3.792B	$3.412B
YoY Change	0.14%	-1.38%	5.06%
Goodwill	$5.761B		$5.762B
YoY Change	-0.02%		0.07%
Intangibles	$2.221B		$3.084B
YoY Change	-27.98%		-12.56%
Long-Term Investments	$2.832B	$2.832B	$3.671B
YoY Change	-22.87%	-22.87%	38.32%
Other Assets	$1.940B	$1.415B	$2.899B
YoY Change	-33.1%	3.29%	131.4%
Total Long-Term Assets	$16.02B	$16.02B	$17.73B
YoY Change	-9.65%	-9.65%	-5.94%
TOTAL ASSETS
Total Short-Term Assets	$7.857B	$7.857B	$6.887B
Total Long-Term Assets	$16.02B	$16.02B	$17.73B
Total Assets	$23.88B	$23.88B	$24.62B
YoY Change	-3.01%	-3.01%	-9.6%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$589.2M	$589.2M	$454.9M
YoY Change	29.52%	29.49%	-14.3%
Accrued Expenses	$1.471B	$1.471B	$1.716B
YoY Change	-14.28%	-14.28%	1.96%
Deferred Revenue
YoY Change
Short-Term Debt	$0.00	$0.00	$0.00
YoY Change
Long-Term Debt Due	$999.1M	$999.1M	$0.00
YoY Change			-100.0%
Total Short-Term Liabilities	$4.298B	$4.298B	$3.742B
YoY Change	14.86%	14.86%	-23.06%
LONG-TERM LIABILITIES
Long-Term Debt	$6.274B	$6.274B	$7.426B
YoY Change	-15.51%	-15.51%	66.54%
Other Long-Term Liabilities	$1.321B	$1.651B	$1.330B
YoY Change	-0.68%	-4.68%	-1.43%
Total Long-Term Liabilities	$1.321B	$7.925B	$1.330B
YoY Change	-0.68%	-13.46%	-1.43%
TOTAL LIABILITIES
Total Short-Term Liabilities	$4.298B	$4.298B	$3.742B
Total Long-Term Liabilities	$1.321B	$7.925B	$1.330B
Total Liabilities	$12.92B	$12.98B	$13.93B
YoY Change	-7.29%	-6.74%	0.27%
SHAREHOLDERS EQUITY
Retained Earnings	$13.91B		$13.98B
YoY Change	-0.46%		-15.07%
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	$10.90B	$10.90B	$10.70B
YoY Change
Total Liabilities & Shareholders Equity	$23.88B	$23.88B	$24.62B
YoY Change	-3.01%	-3.01%	-9.6%

Cashflow Statement

Concept	2021 Q4	2021	2020 Q4
OPERATING ACTIVITIES
Net Income	$368.2M	$1.556B	$357.9M
YoY Change	2.88%	-61.1%	-75.14%
Depreciation, Depletion And Amortization	$134.7M	$487.7M	$90.40M
YoY Change	49.0%	6.67%	-20.0%
Cash From Operating Activities	$838.3M	$3.640B	-$367.1M
YoY Change	-328.36%	-13.95%	-118.73%
INVESTING ACTIVITIES
Capital Expenditures	-$68.50M	$258.1M	-$101.0M
YoY Change	-32.18%	-39.24%	-61.94%
Acquisitions
YoY Change
Other Investing Activities	-$44.20M	-$12.90M	-$65.40M
YoY Change	-32.42%	-147.96%	-106.72%
Cash From Investing Activities	-$112.7M	-$563.7M	-$166.4M
YoY Change	-32.27%	-7.38%	-123.49%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net		$1.800B
YoY Change		-73.05%
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	9.800M	-$2.086B	-401.1M
YoY Change	-102.44%	-60.43%	-81.05%
NET CHANGE
Cash From Operating Activities	838.3M	$3.640B	-367.1M
Cash From Investing Activities	-112.7M	-$563.7M	-166.4M
Cash From Financing Activities	9.800M	-$2.086B	-401.1M
Net Change In Cash	735.4M	$990.0M	-934.6M
YoY Change	-178.69%	-159.95%	-269.25%
FREE CASH FLOW
Cash From Operating Activities	$838.3M	$3.640B	-$367.1M
Capital Expenditures	-$68.50M	$258.1M	-$101.0M
Free Cash Flow	$906.8M	$3.382B	-$266.1M
YoY Change	-440.77%	-11.12%	-111.96%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2021	dei	Entity Central Index Key EntityCentralIndexKey	0000875045
CY2021	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2021
CY2021	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2021	dei	Amendment Flag AmendmentFlag	false
CY2020Q4	us-gaap	Operating Lease Liability Current Statement Of Financial Position Extensible List OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2021-01-31#OtherLiabilitiesCurrent
CY2021Q4	us-gaap	Operating Lease Liability Current Statement Of Financial Position Extensible List OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2021-01-31#OtherLiabilitiesCurrent
CY2021	dei	Document Type DocumentType	10-K
CY2021	dei	Document Annual Report DocumentAnnualReport	true
CY2021	dei	Document Period End Date DocumentPeriodEndDate	2021-12-31
CY2021	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2021	dei	Document Transition Report DocumentTransitionReport	false
CY2021	dei	Entity File Number EntityFileNumber	0-19311
CY2021	dei	Entity Registrant Name EntityRegistrantName	BIOGEN INC.
CY2021	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
CY2021	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	33-0112644
CY2021	dei	Entity Address Address Line1 EntityAddressAddressLine1	225 Binney Street
CY2021	dei	Entity Address City Or Town EntityAddressCityOrTown	Cambridge
CY2021	dei	Entity Address State Or Province EntityAddressStateOrProvince	MA
CY2021	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	02142
CY2021	dei	City Area Code CityAreaCode	617
CY2021	dei	Local Phone Number LocalPhoneNumber	679-2000
CY2021	dei	Security12b Title Security12bTitle	Common Stock, $0.0005 par value
CY2021	dei	Trading Symbol TradingSymbol	BIIB
CY2021	dei	Security Exchange Name SecurityExchangeName	NASDAQ
CY2021	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2021	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2021	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2021	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2021	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2021	dei	Entity Small Business EntitySmallBusiness	false
CY2021	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2021	dei	Icfr Auditor Attestation Flag IcfrAuditorAttestationFlag	true
CY2021	dei	Entity Shell Company EntityShellCompany	false
CY2021Q2	dei	Entity Public Float EntityPublicFloat	51264577902	USD
CY2022Q1	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	146962793	shares
CY2021	dei	Auditor Firm AuditorFirmId	238
CY2021	us-gaap	Revenues Revenues	10981700000	USD
CY2020	us-gaap	Revenues Revenues	13444600000	USD
CY2019	us-gaap	Revenues Revenues	14377900000	USD
CY2021	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	2109700000	USD
CY2020	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	1805200000	USD
CY2019	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	1955400000	USD
CY2021	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2501200000	USD
CY2020	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	3990900000	USD
CY2019	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2280600000	USD
CY2021	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2674300000	USD
CY2020	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2504500000	USD
CY2019	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2374700000	USD
CY2021	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	881300000	USD
CY2020	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	464800000	USD
CY2019	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	489900000	USD
CY2021	biib	Collaboration Profit Sharing CollaborationProfitSharing	7200000	USD
CY2020	biib	Collaboration Profit Sharing CollaborationProfitSharing	232900000	USD
CY2019	biib	Collaboration Profit Sharing CollaborationProfitSharing	241600000	USD
CY2021	us-gaap	Disposal Group Not Discontinued Operation Loss Gain On Write Down DisposalGroupNotDiscontinuedOperationLossGainOnWriteDown	0	USD
CY2020	us-gaap	Disposal Group Not Discontinued Operation Loss Gain On Write Down DisposalGroupNotDiscontinuedOperationLossGainOnWriteDown	-92500000	USD
CY2019	us-gaap	Disposal Group Not Discontinued Operation Loss Gain On Write Down DisposalGroupNotDiscontinuedOperationLossGainOnWriteDown	55300000	USD
CY2021	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	50700000	USD
CY2020	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	86300000	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	63700000	USD
CY2021	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	18000000.0	USD
CY2020	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	75000000.0	USD
CY2019	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	0	USD
CY2021	us-gaap	Restructuring Charges RestructuringCharges	0	USD
CY2020	us-gaap	Restructuring Charges RestructuringCharges	0	USD
CY2019	us-gaap	Restructuring Charges RestructuringCharges	1500000	USD
CY2021	us-gaap	Costs And Expenses CostsAndExpenses	8141000000	USD
CY2020	us-gaap	Costs And Expenses CostsAndExpenses	8894500000	USD
CY2019	us-gaap	Costs And Expenses CostsAndExpenses	7335300000	USD
CY2021	us-gaap	Operating Income Loss OperatingIncomeLoss	2840700000	USD
CY2020	us-gaap	Operating Income Loss OperatingIncomeLoss	4550100000	USD
CY2019	us-gaap	Operating Income Loss OperatingIncomeLoss	7042600000	USD
CY2021	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-1095500000	USD
CY2020	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	497400000	USD
CY2019	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	83300000	USD
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	1745200000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	5047500000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	7125900000	USD
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	52500000	USD
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	992300000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	1158000000	USD
CY2021	biib	Income Loss From Equity Method Investments Net Of Tax IncomeLossFromEquityMethodInvestmentsNetOfTax	34900000	USD
CY2020	biib	Income Loss From Equity Method Investments Net Of Tax IncomeLossFromEquityMethodInvestmentsNetOfTax	5300000	USD
CY2019	biib	Income Loss From Equity Method Investments Net Of Tax IncomeLossFromEquityMethodInvestmentsNetOfTax	-79400000	USD
CY2021	us-gaap	Profit Loss ProfitLoss	1727600000	USD
CY2020	us-gaap	Profit Loss ProfitLoss	4060500000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	5888500000	USD
CY2021	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	171500000	USD
CY2020	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	59900000	USD
CY2019	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	0	USD
CY2021	us-gaap	Net Income Loss NetIncomeLoss	1556100000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	4000600000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	5888500000	USD
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	10.44
CY2020	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	24.86
CY2019	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	31.47
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	10.40
CY2020	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	24.80
CY2019	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	31.42
CY2021	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	149100000	shares
CY2020	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	160900000	shares
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	187100000	shares
CY2021	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	149600000	shares
CY2020	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	161300000	shares
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	187400000	shares
CY2021	us-gaap	Net Income Loss NetIncomeLoss	1556100000	USD
CY2021	us-gaap	Net Income Loss NetIncomeLoss	1556100000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	4000600000	USD
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CY2021	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss After Reclassification And Tax Parent OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent	232800000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss After Reclassification And Tax Parent OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent	-186800000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss After Reclassification And Tax Parent OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent	-26900000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	34000000.0	USD
CY2020	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	-33600000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	21600000	USD
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CY2021	us-gaap	Other Comprehensive Income Loss Foreign Currency Transaction And Translation Adjustment Net Of Tax OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	-92400000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Foreign Currency Transaction And Translation Adjustment Net Of Tax OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	92900000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Foreign Currency Transaction And Translation Adjustment Net Of Tax OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	103800000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	192300000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-163800000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	105200000	USD
CY2021	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	1748400000	USD
CY2020	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	3836800000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	5993700000	USD
CY2021	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	172100000	USD
CY2020	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	60900000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	-400000	USD
CY2021	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	1920500000	USD
CY2020	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	3897700000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	5993300000	USD
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2261400000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	1331200000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Securities Current AvailableForSaleSecuritiesDebtSecuritiesCurrent	1541100000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Securities Current AvailableForSaleSecuritiesDebtSecuritiesCurrent	1278900000	USD
CY2021Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	1549400000	USD
CY2020Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	1913800000	USD
CY2021Q4	biib	Due From Unconsolidated Joint Business DueFromUnconsolidatedJointBusiness	412300000	USD
CY2020Q4	biib	Due From Unconsolidated Joint Business DueFromUnconsolidatedJointBusiness	413500000	USD
CY2021Q4	us-gaap	Inventory Net InventoryNet	1351500000	USD
CY2020Q4	us-gaap	Inventory Net InventoryNet	1068600000	USD
CY2021Q4	us-gaap	Other Assets Current OtherAssetsCurrent	740800000	USD
CY2020Q4	us-gaap	Other Assets Current OtherAssetsCurrent	881100000	USD
CY2021Q4	us-gaap	Assets Current AssetsCurrent	7856500000	USD
CY2020Q4	us-gaap	Assets Current AssetsCurrent	6887100000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Securities Noncurrent AvailableForSaleSecuritiesDebtSecuritiesNoncurrent	892000000.0	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Securities Noncurrent AvailableForSaleSecuritiesDebtSecuritiesNoncurrent	772100000	USD
CY2021Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3416400000	USD
CY2020Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3411500000	USD
CY2021Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	375400000	USD
CY2020Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	433300000	USD
CY2021Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	2221300000	USD
CY2020Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	3084300000	USD
CY2021Q4	us-gaap	Goodwill Goodwill	5761100000	USD
CY2020Q4	us-gaap	Goodwill Goodwill	5762100000	USD
CY2021Q4	us-gaap	Deferred Income Tax Assets Net DeferredIncomeTaxAssetsNet	1415100000	USD
CY2020Q4	us-gaap	Deferred Income Tax Assets Net DeferredIncomeTaxAssetsNet	1369500000	USD
CY2021Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	1939500000	USD
CY2020Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	2899000000	USD
CY2021Q4	us-gaap	Assets Assets	23877300000	USD
CY2020Q4	us-gaap	Assets Assets	24618900000	USD
CY2021Q4	us-gaap	Debt Current DebtCurrent	999100000	USD
CY2020Q4	us-gaap	Debt Current DebtCurrent	0	USD
CY2021Q4	us-gaap	Taxes Payable Current TaxesPayableCurrent	174700000	USD
CY2020Q4	us-gaap	Taxes Payable Current TaxesPayableCurrent	142000000.0	USD
CY2021Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	589200000	USD
CY2020Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	454900000	USD
CY2021Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2535200000	USD
CY2020Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	3145300000	USD
CY2021Q4	us-gaap	Liabilities Current LiabilitiesCurrent	4298200000	USD
CY2020Q4	us-gaap	Liabilities Current LiabilitiesCurrent	3742200000	USD
CY2021Q4	us-gaap	Long Term Debt LongTermDebt	6274000000	USD
CY2020Q4	us-gaap	Long Term Debt LongTermDebt	7426200000	USD
CY2021Q4	us-gaap	Deferred Income Tax Liabilities Net DeferredIncomeTaxLiabilitiesNet	694500000	USD
CY2020Q4	us-gaap	Deferred Income Tax Liabilities Net DeferredIncomeTaxLiabilitiesNet	1032800000	USD
CY2021Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	330400000	USD
CY2020Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	402000000.0	USD
CY2021Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1320500000	USD
CY2020Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1329600000	USD
CY2021Q4	us-gaap	Liabilities Liabilities	12917600000	USD
CY2020Q4	us-gaap	Liabilities Liabilities	13932800000	USD
CY2021Q4	us-gaap	Commitments And Contingencies CommitmentsAndContingencies		USD
CY2020Q4	us-gaap	Commitments And Contingencies CommitmentsAndContingencies		USD
CY2020Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.001
CY2021Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.001
CY2021Q4	us-gaap	Preferred Stock Value PreferredStockValue	0	USD
CY2020Q4	us-gaap	Preferred Stock Value PreferredStockValue	0	USD
CY2021Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0005
CY2020Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0005
CY2021Q4	us-gaap	Common Stock Value CommonStockValue	100000	USD
CY2020Q4	us-gaap	Common Stock Value CommonStockValue	100000	USD
CY2021Q4	us-gaap	Additional Paid In Capital AdditionalPaidInCapital	68200000	USD
CY2020Q4	us-gaap	Additional Paid In Capital AdditionalPaidInCapital	0	USD
CY2021Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-106700000	USD
CY2020Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-299000000.0	USD
CY2021Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	13911700000	USD
CY2020Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	13976300000	USD
CY2021Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	23800000	shares
CY2020Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	23800000	shares
CY2021Q4	us-gaap	Treasury Stock Value TreasuryStockValue	2977100000	USD
CY2020Q4	us-gaap	Treasury Stock Value TreasuryStockValue	2977100000	USD
CY2021Q4	us-gaap	Stockholders Equity StockholdersEquity	10896200000	USD
CY2020Q4	us-gaap	Stockholders Equity StockholdersEquity	10700300000	USD
CY2021Q4	us-gaap	Minority Interest MinorityInterest	63500000	USD
CY2020Q4	us-gaap	Minority Interest MinorityInterest	-14200000	USD
CY2021Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	10959700000	USD
CY2020Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	10686100000	USD
CY2021Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	23877300000	USD
CY2020Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	24618900000	USD
CY2021	us-gaap	Profit Loss ProfitLoss	1727600000	USD
CY2020	us-gaap	Profit Loss ProfitLoss	4060500000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	5888500000	USD
CY2021	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	487700000	USD
CY2020	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	457200000	USD
CY2019	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	464700000	USD
CY2021	us-gaap	Impairment Of Intangible Assets Excluding Goodwill ImpairmentOfIntangibleAssetsExcludingGoodwill	629300000	USD
CY2020	us-gaap	Impairment Of Intangible Assets Excluding Goodwill ImpairmentOfIntangibleAssetsExcludingGoodwill	209700000	USD
CY2019	us-gaap	Impairment Of Intangible Assets Excluding Goodwill ImpairmentOfIntangibleAssetsExcludingGoodwill	215900000	USD
CY2021	us-gaap	Inventory Write Down InventoryWriteDown	167600000	USD
CY2020	us-gaap	Inventory Write Down InventoryWriteDown	26600000	USD
CY2019	us-gaap	Inventory Write Down InventoryWriteDown	52200000	USD
CY2021	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	18000000.0	USD
CY2019	us-gaap	Increase Decrease In Equity Securities Fv Ni IncreaseDecreaseInEquitySecuritiesFvNi	147300000	USD
CY2019	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	0	USD
CY2020	us-gaap	Research And Development In Process ResearchAndDevelopmentInProcess	75000000.0	USD
CY2021	us-gaap	Share Based Compensation ShareBasedCompensation	238600000	USD
CY2020	us-gaap	Share Based Compensation ShareBasedCompensation	198300000	USD
CY2019	us-gaap	Share Based Compensation ShareBasedCompensation	182300000	USD
CY2021	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	50700000	USD
CY2020	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	86300000	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	63700000	USD
CY2021	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
CY2020	us-gaap	Income Loss From Individually Significant Component Disposed Of Or Held For Sale Excluding Discontinued Operations Attributable To Parent Before Income Tax IncomeLossFromIndividuallySignificantComponentDisposedOfOrHeldForSaleExcludingDiscontinuedOperationsAttributableToParentBeforeIncomeTax	92500000	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
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-426800000	USD
CY2020	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	149000000.0	USD
CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	67100000	USD
CY2021	us-gaap	Increase Decrease In Equity Securities Fv Ni IncreaseDecreaseInEquitySecuritiesFvNi	-826800000	USD
CY2020	us-gaap	Increase Decrease In Equity Securities Fv Ni IncreaseDecreaseInEquitySecuritiesFvNi	681800000	USD
CY2021	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	34900000	USD
CY2020	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	3300000	USD
CY2019	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	-77400000	USD
CY2021	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-202200000	USD
CY2020	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-104600000	USD
CY2019	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-86900000	USD
CY2021	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-324800000	USD
CY2020	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-2800000	USD
CY2019	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-68800000	USD
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CY2020	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-176700000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	63300000	USD
CY2021	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	462400000	USD
CY2020	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	316300000	USD
CY2019	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	19200000	USD
CY2021	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-95400000	USD
CY2020	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	154200000	USD
CY2019	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	240200000	USD
CY2021	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	230800000	USD
CY2020	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	-67500000	USD
CY2019	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	16100000	USD
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CY2020	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-137100000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-43300000	USD
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CY2020	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	4229800000	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	7078600000	USD
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CY2020	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	424800000	USD
CY2019	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	514500000	USD
CY2021	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	3405400000	USD
CY2020	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	7299400000	USD
CY2019	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	6007000000	USD
CY2021	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	3808700000	USD
CY2020	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	6397700000	USD
CY2019	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	5252600000	USD
CY2021	us-gaap	Payments To Acquire Other Productive Assets PaymentsToAcquireOtherProductiveAssets	0	USD
CY2020	us-gaap	Payments To Acquire Other Productive Assets PaymentsToAcquireOtherProductiveAssets	0	USD
CY2019	us-gaap	Payments To Acquire Other Productive Assets PaymentsToAcquireOtherProductiveAssets	300000000.0	USD
CY2021	us-gaap	Payments To Acquire Business Two Net Of Cash Acquired PaymentsToAcquireBusinessTwoNetOfCashAcquired	0	USD
CY2020	us-gaap	Payments To Acquire Business Two Net Of Cash Acquired PaymentsToAcquireBusinessTwoNetOfCashAcquired	0	USD
CY2019	us-gaap	Payments To Acquire Business Two Net Of Cash Acquired PaymentsToAcquireBusinessTwoNetOfCashAcquired	744400000	USD
CY2021	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	28100000	USD
CY2020	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	USD
CY2019	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	923700000	USD
CY2021	us-gaap	Payments To Acquire In Process Research And Development PaymentsToAcquireInProcessResearchAndDevelopment	18000000.0	USD
CY2020	us-gaap	Payments To Acquire In Process Research And Development PaymentsToAcquireInProcessResearchAndDevelopment	75000000.0	USD
CY2019	us-gaap	Payments To Acquire In Process Research And Development PaymentsToAcquireInProcessResearchAndDevelopment	0	USD
CY2021	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	18800000	USD
CY2020	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	52000000.0	USD
CY2019	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	155000000.0	USD
CY2021	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	93500000	USD
CY2020	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	74900000	USD
CY2019	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	479300000	USD
CY2021	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-12900000	USD
CY2020	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	26900000	USD
CY2019	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-27000000.0	USD
CY2021	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-563700000	USD
CY2020	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-608600000	USD
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	470500000	USD
CY2021	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	1800000000	USD
CY2020	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	6679100000	USD
CY2019	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	5868300000	USD
CY2021	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	700000	USD
CY2020	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	4600000	USD
CY2019	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	0	USD
CY2021	biib	Repayments Of Debt Including Premiums Paid RepaymentsOfDebtIncludingPremiumsPaid	170000000.0	USD
CY2020	biib	Repayments Of Debt Including Premiums Paid RepaymentsOfDebtIncludingPremiumsPaid	0	USD
CY2019	biib	Repayments Of Debt Including Premiums Paid RepaymentsOfDebtIncludingPremiumsPaid	0	USD
CY2021	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	0	USD
CY2020	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	2967400000	USD
CY2019	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	0	USD
CY2021	us-gaap	Repayments Of Debt RepaymentsOfDebt	0	USD
CY2020	us-gaap	Repayments Of Debt RepaymentsOfDebt	1500000000	USD
CY2019	us-gaap	Repayments Of Debt RepaymentsOfDebt	0	USD
CY2021	us-gaap	Proceeds From Payments To Minority Shareholders ProceedsFromPaymentsToMinorityShareholders	-94400000	USD
CY2020	us-gaap	Proceeds From Payments To Minority Shareholders ProceedsFromPaymentsToMinorityShareholders	-71000000.0	USD
CY2019	us-gaap	Proceeds From Payments To Minority Shareholders ProceedsFromPaymentsToMinorityShareholders	4300000	USD
CY2021	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-21100000	USD
CY2020	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	14600000	USD
CY2019	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	3600000	USD
CY2021	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-2086200000	USD
CY2020	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-5272700000	USD
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-5860400000	USD
CY2021	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	990000000.0	USD
CY2020	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	-1651500000	USD
CY2019	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	1688700000	USD
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CY2020	us-gaap	Effect Of Exchange Rate On Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	69000000.0	USD
CY2019	us-gaap	Effect Of Exchange Rate On Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	400000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	1331200000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2913700000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	1224600000	USD
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2261400000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	1331200000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2913700000	USD
CY2020Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	10686100000	USD
CY2021	us-gaap	Profit Loss ProfitLoss	1727600000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	192900000	USD
CY2021	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	100000000.0	USD
CY2021	biib	Noncontrolling Interest Increase Decrease Other NoncontrollingInterestIncreaseDecreaseOther	5600000	USD
CY2021	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	54400000	USD
CY2021	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	-55000000.0	USD
CY2021	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	-246600000	USD
CY2021	us-gaap	Stockholders Equity Other StockholdersEquityOther	-1500000	USD
CY2021Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	10959700000	USD
CY2019Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13339100000	USD
CY2020	us-gaap	Profit Loss ProfitLoss	4060500000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-162800000	USD
CY2020	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	75000000.0	USD
CY2020	biib	Noncontrolling Interest Increase Decrease Other NoncontrollingInterestIncreaseDecreaseOther	4000000.0	USD
CY2020	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	49300000	USD
CY2020	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	-53700000	USD
CY2020	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	-204500000	USD
CY2020	us-gaap	Stockholders Equity Other StockholdersEquityOther	700000	USD
CY2020Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	10686100000	USD
CY2018Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13031600000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	5888500000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	104800000	USD
CY2019	biib	Noncontrolling Interest Increase Decrease Other 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 Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	-53800000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	-191200000	USD
CY2019Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13339100000	USD
CY2021	biib	Interest In Subsidiary InterestInSubsidiary	1.000
CY2021	us-gaap	Use Of Estimates UseOfEstimates	<div style="margin-top:9pt"><span style="color:#2573ba;font-family:'Franklin Gothic Book',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:120%">Use of Estimates</span></div><div style="margin-top:6pt;text-indent:24.75pt"><span style="color:#000000;font-family:'Franklin Gothic Book',sans-serif;font-size:10pt;font-weight:400;line-height:120%">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, revenue and expense and related </span></div><div style="margin-top:6pt"><span style="color:#000000;font-family:'Franklin Gothic Book',sans-serif;font-size:10pt;font-weight:400;line-height:120%">disclosure of contingent assets and liabilities. On an ongoing basis we evaluate our estimates, judgments and assumptions. 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 revenue and expense. Actual results may differ from these estimates.</span></div><div style="margin-top:6pt;text-indent:24.75pt"><span style="color:#000000;font-family:'Franklin Gothic Book',sans-serif;font-size:10pt;font-weight:400;line-height:120%">The length of time and full extent to which the COVID-19 pandemic directly or indirectly impacts our business, results of operations and financial condition, including sales, expense, reserves and allowances, the supply chain, manufacturing, clinical trials, research and development costs and employee-related costs, depends on future developments that are highly uncertain, subject to change and are difficult to predict, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat COVID-19 as well as the economic impact on local, regional, national and international customers and markets. We have made estimates of the impact of the COVID-19 pandemic within our consolidated financial statements and there may be changes to those estimates in future periods.</span></div>
CY2021	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	<div style="margin-top:9pt"><span style="color:#2573ba;font-family:'Franklin Gothic Book',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:120%">Concentration of Credit Risk</span></div><div style="margin-top:6pt;text-indent:24.75pt"><span style="color:#000000;font-family:'Franklin Gothic Book',sans-serif;font-size:10pt;font-weight:400;line-height:120%">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>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.
CY2021	us-gaap	Number Of Operating Segments NumberOfOperatingSegments	1	segment
CY2021	us-gaap	Advertising Expense AdvertisingExpense	98700000	USD
CY2020	us-gaap	Advertising Expense AdvertisingExpense	111800000	USD
CY2019	us-gaap	Advertising Expense AdvertisingExpense	79200000	USD
CY2021	biib	Number Of Wholesalers NumberOfWholesalers	2	wholesaler
CY2021	biib	Number Of Wholesalers NumberOfWholesalers	2	wholesaler
CY2020Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1276000000	USD
CY2021	biib	Current Provisions To Product Revenue Reserves Relating To Sales In Current Year CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear	3700600000	USD
CY2021	biib	Adjustments To Product Revenue Reserves Relating To Sales In Prior Years AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears	103400000	USD
CY2021	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Current Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear	2882800000	USD
CY2021	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear	1055100000	USD
CY2021Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	935300000	USD
CY2019Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1198900000	USD
CY2020	biib	Current Provisions To Product Revenue Reserves Relating To Sales In Current Year CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear	4102500000	USD
CY2020	biib	Adjustments To Product Revenue Reserves Relating To Sales In Prior Years AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears	53700000	USD
CY2020	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Current Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear	3061200000	USD
CY2020	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear	910500000	USD
CY2020Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1276000000	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
CY2021Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	935300000	USD
CY2020Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1276000000	USD
CY2021	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.151
CY2020	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.147
CY2019	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.159
CY2021Q4	us-gaap	Inventory Raw Materials Net Of Reserves InventoryRawMaterialsNetOfReserves	349600000	USD
CY2020Q4	us-gaap	Inventory Raw Materials Net Of Reserves InventoryRawMaterialsNetOfReserves	314900000	USD
CY2021Q4	us-gaap	Inventory Work In Process Net Of Reserves InventoryWorkInProcessNetOfReserves	814000000.0	USD
CY2020Q4	us-gaap	Inventory Work In Process Net Of Reserves InventoryWorkInProcessNetOfReserves	544500000	USD
CY2021Q4	us-gaap	Inventory Finished Goods Net Of Reserves InventoryFinishedGoodsNetOfReserves	187900000	USD
CY2020Q4	us-gaap	Inventory Finished Goods Net Of Reserves InventoryFinishedGoodsNetOfReserves	209200000	USD
CY2021Q4	biib	Inventory Net Current And Noncurrent InventoryNetCurrentAndNoncurrent	1351500000	USD
CY2020Q4	biib	Inventory Net Current And Noncurrent InventoryNetCurrentAndNoncurrent	1068600000	USD
CY2021	us-gaap	Inventory Write Down InventoryWriteDown	167600000	USD
CY2020	us-gaap	Inventory Write Down InventoryWriteDown	26600000	USD
CY2019	us-gaap	Inventory Write Down InventoryWriteDown	52200000	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	7413100000	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5388500000	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	2024600000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	7394300000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5136500000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	2257800000	USD
CY2021Q4	us-gaap	Intangible Assets Gross Excluding Goodwill IntangibleAssetsGrossExcludingGoodwill	7609800000	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5388500000	USD
CY2021Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	2221300000	USD
CY2020Q4	us-gaap	Intangible Assets Gross Excluding Goodwill IntangibleAssetsGrossExcludingGoodwill	8220800000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5136500000	USD
CY2020Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	3084300000	USD
CY2021	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	881300000	USD
CY2020	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	464800000	USD
CY2019	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	489900000	USD
CY2021	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	252000000	USD
CY2020	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	255100000	USD
CY2019	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	274000000	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Next Twelve Months FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths	255000000.0	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Two FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo	210000000.0	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Three FiniteLivedIntangibleAssetsAmortizationExpenseYearThree	195000000.0	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Four FiniteLivedIntangibleAssetsAmortizationExpenseYearFour	195000000.0	USD
CY2021Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Five FiniteLivedIntangibleAssetsAmortizationExpenseYearFive	180000000.0	USD
CY2020Q4	us-gaap	Goodwill Goodwill	5762100000	USD
CY2021	us-gaap	Goodwill Other Increase Decrease GoodwillOtherIncreaseDecrease	-1000000.0	USD
CY2020	us-gaap	Goodwill Other Increase Decrease GoodwillOtherIncreaseDecrease	4300000	USD
CY2021Q4	us-gaap	Goodwill Goodwill	5761100000	USD
CY2019Q4	us-gaap	Goodwill Goodwill	5757800000	USD
CY2020Q4	us-gaap	Goodwill Goodwill	5762100000	USD
CY2021Q4	us-gaap	Goodwill Impaired Accumulated Impairment Loss GoodwillImpairedAccumulatedImpairmentLoss	0	USD
CY2021	biib	Technological And Regulatory Success Probability TechnologicalAndRegulatorySuccessProbability	0.109
CY2021Q4	us-gaap	Cash Equivalents At Carrying Value CashEquivalentsAtCarryingValue	1632200000	USD
CY2020Q4	us-gaap	Cash Equivalents At Carrying Value CashEquivalentsAtCarryingValue	626900000	USD
CY2021Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2435900000	USD
CY2021Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	400000	USD
CY2021Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	3200000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2433100000	USD
CY2021Q4	us-gaap	Equity Securities Fv Ni Cost EquitySecuritiesFvNiCost	1167000000	USD
CY2021Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Gain Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTax	160900000	USD
CY2021Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Loss Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTax	279400000	USD
CY2021Q4	us-gaap	Equity Securities Fv Ni EquitySecuritiesFvNi	1048500000	USD
CY2020Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2049100000	USD
CY2020Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	2300000	USD
CY2020Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	400000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2051000000	USD
CY2020Q4	us-gaap	Equity Securities Fv Ni Cost EquitySecuritiesFvNiCost	1239500000	USD
CY2020Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Gain Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTax	749700000	USD
CY2020Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Loss Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTax	14900000	USD
CY2020Q4	us-gaap	Equity Securities Fv Ni EquitySecuritiesFvNi	1974300000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Fair Value AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue	1541100000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost	1541700000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Fair Value AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue	1278900000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost	1278600000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	868200000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost	870200000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	722600000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost	721300000	USD
CY2021Q4	biib	Available For Sale Securities Debt Maturities After Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsFairValue	23800000	USD
CY2021Q4	biib	Available For Sale Securities Debt Maturities After Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsAmortizedCost	24000000.0	USD
CY2020Q4	biib	Available For Sale Securities Debt Maturities After Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsFairValue	49500000	USD
CY2020Q4	biib	Available For Sale Securities Debt Maturities After Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsAmortizedCost	49200000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2433100000	USD
CY2021Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2435900000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2051000000	USD
CY2020Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2049100000	USD
CY2021	biib	Average Maturity Of Marketable Securities AverageMaturityOfMarketableSecurities	P10M
CY2020	biib	Average Maturity Of Marketable Securities AverageMaturityOfMarketableSecurities	P11M
CY2021	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	3405400000	USD
CY2020	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	7299400000	USD
CY2019	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	6007000000	USD
CY2021	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	200000	USD
CY2020	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	17700000	USD
CY2019	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	6000000.0	USD
CY2021	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	4000000.0	USD
CY2020	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	26000000.0	USD
CY2019	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	1500000	USD
CY2021	us-gaap	Derivative Remaining Maturity1 DerivativeRemainingMaturity1	P15M
CY2021	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	-43300000	USD
CY2020	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	30100000	USD
CY2019	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	-5900000	USD
CY2021Q4	us-gaap	Land Land	207500000	USD
CY2020Q4	us-gaap	Land Land	119800000	USD
CY2021Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	1699700000	USD
CY2020Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	1025300000	USD
CY2021Q4	us-gaap	Leasehold Improvements Gross LeaseholdImprovementsGross	121000000.0	USD
CY2020Q4	us-gaap	Leasehold Improvements Gross LeaseholdImprovementsGross	104600000	USD
CY2021Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	1585500000	USD
CY2020Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	1027800000	USD
CY2021Q4	biib	Computer Software And Hardware ComputerSoftwareAndHardware	971600000	USD
CY2020Q4	biib	Computer Software And Hardware ComputerSoftwareAndHardware	903000000.0	USD
CY2021Q4	us-gaap	Furniture And Fixtures Gross FurnitureAndFixturesGross	67400000	USD
CY2020Q4	us-gaap	Furniture And Fixtures Gross FurnitureAndFixturesGross	62500000	USD
CY2021Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	770300000	USD
CY2020Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	1950800000	USD
CY2021Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	5423000000	USD
CY2020Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	5193800000	USD
CY2021Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	2006600000	USD
CY2020Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	1782300000	USD
CY2021Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3416400000	USD
CY2020Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3411500000	USD
CY2021	us-gaap	Depreciation Depreciation	235300000	USD
CY2020	us-gaap	Depreciation Depreciation	201900000	USD
CY2019	us-gaap	Depreciation Depreciation	190600000	USD
CY2021	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	36300000	USD
CY2020	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	65200000	USD
CY2019	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	68800000	USD
CY2021Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	419500000	USD
CY2020Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	485200000	USD
CY2021	us-gaap	Lease Cost LeaseCost	97900000	USD
CY2020	us-gaap	Lease Cost LeaseCost	92400000	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	99600000	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	89700000	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	79700000	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	57200000	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Five LesseeOperatingLeaseLiabilityPaymentsDueYearFive	44800000	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Payments Due After Year Five LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive	84000000.0	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	455000000.0	USD
CY2021Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	35500000	USD
CY2021Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	419500000	USD
CY2021Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P5Y5M4D
CY2020Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P6Y3M18D
CY2021Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.029
CY2020Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.029
CY2021	us-gaap	Operating Lease Payments OperatingLeasePayments	105800000	USD
CY2020	us-gaap	Operating Lease Payments OperatingLeasePayments	100200000	USD
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	18100000	USD
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	59000000.0	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	1000000000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	0	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	0	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	1750000000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive	0	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal After Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive	4817300000	USD
CY2021Q4	us-gaap	Long Term Notes Payable LongTermNotesPayable	7567300000	USD
CY2021Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	8000000	shares
CY2021Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2019Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2020Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2020Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2021Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2019Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2021Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000	shares
CY2021Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	170800000	shares
CY2021Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	147000000.0	shares
CY2020Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000	shares
CY2020Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	176200000	shares
CY2020Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	152400000	shares
CY2019Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000	shares
CY2019Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	198000000.0	shares
CY2019Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	174200000	shares
CY2021	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	134100000	USD
CY2021	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	-58200000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	192300000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	-145600000	USD
CY2020	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	18200000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-163800000	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
CY2021	us-gaap	Net Income Loss NetIncomeLoss	1556100000	USD
CY2021	us-gaap	Net Income Loss NetIncomeLoss	1556100000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	4000600000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	5888500000	USD
CY2021	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	149100000	shares
CY2020	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	160900000	shares
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	187100000	shares
CY2021	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	500000	shares
CY2020	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	400000	shares
CY2019	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	300000	shares
CY2021	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	149600000	shares
CY2020	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	161300000	shares
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	187400000	shares
CY2021	us-gaap	Weighted Average Number Of Shares Common Stock Subject To Repurchase Or Cancellation WeightedAverageNumberOfSharesCommonStockSubjectToRepurchaseOrCancellation	6000000	shares
CY2020	us-gaap	Weighted Average Number Of Shares Common Stock Subject To Repurchase Or Cancellation WeightedAverageNumberOfSharesCommonStockSubjectToRepurchaseOrCancellation	22400000	shares
CY2019	us-gaap	Weighted Average Number Of Shares Common Stock Subject To Repurchase Or Cancellation WeightedAverageNumberOfSharesCommonStockSubjectToRepurchaseOrCancellation	23600000	shares
CY2021	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	258800000	USD
CY2020	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	211300000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	225400000	USD
CY2021	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8000000.0	USD
CY2020	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	6200000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8900000	USD
CY2021	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	250800000	USD
CY2020	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	205100000	USD
CY2019	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	216500000	USD
CY2021	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	46700000	USD
CY2020	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	33500000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	35700000	USD
CY2021	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	258800000	USD
CY2020	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	211300000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	225400000	USD
CY2021	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8000000.0	USD
CY2020	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	6200000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8900000	USD
CY2021	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	250800000	USD
CY2020	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	205100000	USD
CY2019	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	216500000	USD
CY2021Q4	us-gaap	Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized	235500000	USD
CY2021	biib	Weighted Average Period To Recognize Cost Of Nonvested Awards WeightedAveragePeriodToRecognizeCostOfNonvestedAwards	P1Y10M24D
CY2021	biib	Number Of Share Based Compensation Plans Pursuant To Which Awards Are Currently Being Made NumberOfShareBasedCompensationPlansPursuantToWhichAwardsAreCurrentlyBeingMade	3	plan
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	2900000	USD
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	4200000	USD
CY2020	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	2900000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	2500000	USD
CY2020	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	700000	USD
CY2019	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	400000	USD
CY2015Q2	us-gaap	Stock Issued During Period Shares Employee Stock Purchase Plans StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans	6200000	shares
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	448300000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	3290000000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	4725300000	USD
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	1296900000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	1757500000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	2400600000	USD
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	1745200000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	5047500000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	7125900000	USD
CY2021	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	319100000	USD
CY2020	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	647000000.0	USD
CY2019	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	947400000	USD
CY2021	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	23100000	USD
CY2020	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	41200000	USD
CY2019	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	59100000	USD
CY2021	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	137100000	USD
CY2020	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	155100000	USD
CY2019	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	84400000	USD
CY2021	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	479300000	USD
CY2020	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	843300000	USD
CY2021	us-gaap	Income Taxes Paid IncomeTaxesPaid	72700000	USD
CY2019	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	1090900000	USD
CY2021	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-242500000	USD
CY2020	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-1749900000	USD
CY2019	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	1143900000	USD
CY2021	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-11900000	USD
CY2020	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-6800000	USD
CY2019	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-2300000	USD
CY2021	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-172400000	USD
CY2020	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	1905700000	USD
CY2019	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-1074500000	USD
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-426800000	USD
CY2020	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	149000000.0	USD
CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	67100000	USD
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	52500000	USD
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	992300000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	1158000000	USD
CY2021Q4	biib	Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries	1500000000	USD
CY2021Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	121000000.0	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	113400000	USD
CY2021Q4	us-gaap	Deferred Tax Assets Inventory DeferredTaxAssetsInventory	199400000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Inventory DeferredTaxAssetsInventory	165900000	USD
CY2021Q4	us-gaap	Deferred Tax Assets Goodwill And Intangible Assets DeferredTaxAssetsGoodwillAndIntangibleAssets	1477500000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Goodwill And Intangible Assets DeferredTaxAssetsGoodwillAndIntangibleAssets	1546000000	USD
CY2021Q4	biib	Deferred Tax Assets Counterparty Tax Basis In Asset DeferredTaxAssetsCounterpartyTaxBasisInAsset	475800000	USD
CY2020Q4	biib	Deferred Tax Assets Counterparty Tax Basis In Asset DeferredTaxAssetsCounterpartyTaxBasisInAsset	0	USD
CY2021Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	1973000000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	2080300000	USD
CY2021Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	31700000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	23300000	USD
CY2021Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	208800000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	103100000	USD
CY2021Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	1961300000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	1753900000	USD
CY2021Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2525900000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2278100000	USD
CY2021Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets	256600000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets	396200000	USD
CY2021Q4	biib	Deferredtaxliabilities Gilt Itaxcalculation DeferredtaxliabilitiesGILTItaxcalculation	1037600000	USD
CY2020Q4	biib	Deferredtaxliabilities Gilt Itaxcalculation DeferredtaxliabilitiesGILTItaxcalculation	1143700000	USD
CY2021Q4	biib	Deferred Tax Liabilities Tax Credit Carryforwards DeferredTaxLiabilitiesTaxCreditCarryforwards	260200000	USD
CY2020Q4	biib	Deferred Tax Liabilities Tax Credit Carryforwards DeferredTaxLiabilitiesTaxCreditCarryforwards	174600000	USD
CY2021Q4	biib	Deferred Tax Liabilities Depreciation Amortization Other DeferredTaxLiabilitiesDepreciationAmortizationOther	250900000	USD
CY2020Q4	biib	Deferred Tax Liabilities Depreciation Amortization Other DeferredTaxLiabilitiesDepreciationAmortizationOther	227000000.0	USD
CY2021Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	1805300000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	1941500000	USD
CY2021Q4	biib	Totaldeferredchargesandprepaidtaxes Totaldeferredchargesandprepaidtaxes	39600000	USD
CY2020Q4	biib	Totaldeferredchargesandprepaidtaxes Totaldeferredchargesandprepaidtaxes	142200000	USD
CY2021	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2020	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2019	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2021	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.008
CY2020	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.007
CY2019	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.008
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.105
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.033
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.045
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.038
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.012
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.011
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	-0.016
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	0.007
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	0.004
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Assets EffectiveIncomeTaxRateReconciliationDispositionOfAssets	0
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Assets EffectiveIncomeTaxRateReconciliationDispositionOfAssets	-0.004
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Assets EffectiveIncomeTaxRateReconciliationDispositionOfAssets	0.010
CY2021	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	0
CY2020	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	0
CY2019	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	-0.021
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Impairment Losses EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses	0
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Impairment Losses EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses	0.018
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Impairment Losses EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses	0
CY2021	biib	Effective Tax Rate Reconciliation Gilt Itax EffectiveTaxRateReconciliationGILTItax	0.013
CY2020	biib	Effective Tax Rate Reconciliation Gilt Itax EffectiveTaxRateReconciliationGILTItax	0.013
CY2019	biib	Effective Tax Rate Reconciliation Gilt Itax EffectiveTaxRateReconciliationGILTItax	0.015
CY2021	biib	Effective Income Tax Rate Reconciliation Changein Foreign Tax Rate Percent EffectiveIncomeTaxRateReconciliationChangeinForeignTaxRatePercent	0
CY2020	biib	Effective Income Tax Rate Reconciliation Changein Foreign Tax Rate Percent EffectiveIncomeTaxRateReconciliationChangeinForeignTaxRatePercent	0
CY2019	biib	Effective Income Tax Rate Reconciliation Changein Foreign Tax Rate Percent EffectiveIncomeTaxRateReconciliationChangeinForeignTaxRatePercent	-0.008
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Research And Development EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment	0.053
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Research And Development EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment	0.001
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Research And Development EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment	0
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0.011
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	-0.008
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0.001
CY2021	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.030
CY2020	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.197
CY2019	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.163
CY2021Q2	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	490000000	USD
CY2021Q2	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	945000000	USD
CY2021Q2	us-gaap	Unrecognized Tax Benefits Period Increase Decrease UnrecognizedTaxBenefitsPeriodIncreaseDecrease	455000000	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	75700000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	129900000	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	114200000	USD
CY2021	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	4200000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	1500000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	5300000	USD
CY2021	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	509900000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	51700000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	17200000	USD
CY2021	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	18800000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	63600000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	10300000	USD
CY2021	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	3200000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	7900000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	100000	USD
CY2021	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	4400000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	35900000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsIncreasesResultingFromSettlementsWithTaxingAuthorities	3600000	USD
CY2021Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	563400000	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	75700000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	129900000	USD
CY2021Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	87500000	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	68800000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	122700000	USD
CY2021	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	2700000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	1000000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	4700000	USD
CY2021Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	24800000	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	21200000	USD
CY2021Q4	us-gaap	Decrease In Unrecognized Tax Benefits Is Reasonably Possible DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible	475000000	USD
CY2019Q4	us-gaap	Loss Contingency Damages Sought Value LossContingencyDamagesSoughtValue	200000000	USD
CY2020Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1329600000	USD
CY2021Q4	us-gaap	Accrued Income Taxes Noncurrent AccruedIncomeTaxesNoncurrent	664500000	USD
CY2021	us-gaap	Interest Paid Net InterestPaidNet	280800000	USD
CY2020	us-gaap	Interest Paid Net InterestPaidNet	272700000	USD
CY2019	us-gaap	Interest Paid Net InterestPaidNet	244200000	USD
CY2021	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	247900000	USD
CY2020	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	906700000	USD
CY2019	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	1064500000	USD
CY2021	us-gaap	Investment Income Interest InvestmentIncomeInterest	11000000.0	USD
CY2020	us-gaap	Investment Income Interest InvestmentIncomeInterest	42000000.0	USD
CY2019	us-gaap	Investment Income Interest InvestmentIncomeInterest	120000000.0	USD
CY2021	us-gaap	Interest Expense InterestExpense	253600000	USD
CY2020	us-gaap	Interest Expense InterestExpense	222500000	USD
CY2019	us-gaap	Interest Expense InterestExpense	187400000	USD
CY2021	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	-824900000	USD
CY2020	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	685700000	USD
CY2019	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	204700000	USD
CY2021	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	-22400000	USD
CY2020	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	-10700000	USD
CY2019	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	-7000000.0	USD
CY2021	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	-5600000	USD
CY2020	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	2900000	USD
CY2019	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	-47000000.0	USD
CY2021	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-1095500000	USD
CY2020	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	497400000	USD
CY2019	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	83300000	USD
CY2021	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	-821100000	USD
CY2020	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	693900000	USD
CY2019	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	200100000	USD
CY2021	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	10300000	USD
CY2020	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	12100000	USD
CY2019	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	50000000.0	USD
CY2021	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	-831400000	USD
CY2020	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	681800000	USD
CY2019	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	150100000	USD
CY2021Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	1320500000	USD
CY2020Q4	us-gaap	Accrued Income Taxes Noncurrent AccruedIncomeTaxesNoncurrent	709900000	USD
CY2019	biib	Number Of Ophthalmolgy Biosimilar Products Exclusive Rights NumberOfOphthalmolgyBiosimilarProductsExclusiveRights	2	product
CY2021Q2	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	490000000	USD
CY2021Q2	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	945000000	USD
CY2021Q2	us-gaap	Unrecognized Tax Benefits Period Increase Decrease UnrecognizedTaxBenefitsPeriodIncreaseDecrease	455000000	USD
CY2021Q4	biib	Investment In Biotechnology Companies That Are Determined To Be Unconsolidated Variable Interest Entities InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities	24600000	USD
CY2020Q4	biib	Investment In Biotechnology Companies That Are Determined To Be Unconsolidated Variable Interest Entities InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities	12800000	USD
CY2021Q3	us-gaap	Loss Contingency Estimate Of Possible Loss LossContingencyEstimateOfPossibleLoss	981100000	USD
CY2019Q1	us-gaap	Payment For Contingent Consideration Liability Financing Activities PaymentForContingentConsiderationLiabilityFinancingActivities	300000000	USD
CY2021Q4	biib	Potential Future Milestone Payments Commitment To Third Party Approximately PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately	10000000000	USD
CY2021Q4	biib	Accrued Expenses On Funding Commitment Related To Clinical Research AccruedExpensesOnFundingCommitmentRelatedToClinicalResearch	27300000	USD
CY2021Q4	us-gaap	Other Commitment OtherCommitment	676100000	USD
CY2021Q4	us-gaap	Liability For Uncertain Tax Positions Current LiabilityForUncertainTaxPositionsCurrent	106800000	USD
CY2021	us-gaap	Income Taxes Paid IncomeTaxesPaid	72700000	USD
CY2021	us-gaap	Number Of Reportable Segments NumberOfReportableSegments	1	segment
CY2021Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3791800000	USD
CY2020Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3844800000	USD
CY2019Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3674300000	USD
CY2021	dei	Auditor Name AuditorName	PricewaterhouseCoopers LLP
CY2021	dei	Auditor Location AuditorLocation	Boston, Massachusetts

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2021 Q4 Form 10-K Financial Statement

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$BIIB BIOGEN INC. Stock $biib

2021 Q4 Form 10-K Financial Statement

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$BIIB
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