$BIIB
BIOGEN INC. Stock $biib

$biib Overview Financials Valuation SEC Filings Company Website

2024 Q1 Form 10-K Financial Statement

#000087504524000009 Filed on February 14, 2024

Income Statement

Concept	2024 Q1	2023 Q4	2023
Revenue	$2.291B	$2.386B	$9.836B
YoY Change	-7.0%	-6.2%	-3.32%
Cost Of Revenue	$542.2M	$618.3M	$2.533B
YoY Change	-18.2%	8.3%	11.2%
Gross Profit	$1.748B	$1.768B	$7.302B
YoY Change	-2.88%	-10.39%	-7.51%
Gross Profit Margin	76.33%	74.09%	74.24%
Selling, General & Admin	$578.3M	$595.1M	$2.550B
YoY Change	-4.41%	-5.96%	6.08%
% of Gross Profit	33.08%	33.66%	34.92%
Research & Development	$445.3M	$568.1M	$2.462B
YoY Change	-21.96%	-5.57%	10.35%
% of Gross Profit	25.47%	32.13%	33.72%
Depreciation & Amortization	$147.5M	$76.60M	$494.8M
YoY Change	31.34%	-36.06%	-4.55%
% of Gross Profit	8.44%	4.33%	6.78%
Operating Expenses	$1.102B	$1.240B	$5.012B
YoY Change	-6.27%	13.29%	8.13%
Operating Profit	$646.4M	$528.2M	$2.291B
YoY Change	3.49%	-39.89%	-29.75%
Interest Expense	-$157.4M	-$55.80M	$246.9M
YoY Change	-351.84%	-61.89%	0.12%
% of Operating Profit	-24.35%	-10.56%	10.78%
Other Income/Expense, Net	-$1.900M	-$65.80M	-$315.5M
YoY Change	-416.67%	-535.76%	-391.59%
Pretax Income	$464.8M	$291.6M	$192.4M
YoY Change	6.05%	-51.71%	-89.55%
Income Tax	$71.40M	$42.70M	$135.3M
% Of Pretax Income	15.36%	14.64%	70.32%
Net Earnings	$393.4M	$249.7M	$1.161B
YoY Change	1.42%	-54.63%	-61.89%
Net Earnings / Revenue	17.18%	10.46%	11.81%
Basic Earnings Per Share	$2.71		$8.02
Diluted Earnings Per Share	$2.70	$1.71	$7.97
COMMON SHARES
Basic Shares Outstanding	145.4M shares	144.9M shares	144.7M shares
Diluted Shares Outstanding	145.9M shares		145.6M shares

Balance Sheet

Concept	2024 Q1	2023 Q4	2023
SHORT-TERM ASSETS
Cash & Short-Term Investments	$1.074B	$1.050B	$1.050B
YoY Change	-78.69%	-78.54%	-78.54%
Cash & Equivalents	$1.074B	$1.050B	$1.050B
Short-Term Investments		$0.00	$0.00
Other Short-Term Assets	$1.165B	$1.182B	$1.182B
YoY Change	-17.47%	-16.62%	-16.62%
Inventory	$2.517B	$2.527B	$2.527B
Prepaid Expenses
Receivables	$2.000B	$1.664B	$2.100B
Other Receivables	$0.00	$0.00	$0.00
Total Short-Term Assets	$6.756B	$6.859B	$6.859B
YoY Change	-30.79%	-29.94%	-29.94%
LONG-TERM ASSETS
Property, Plant & Equipment	$3.703B	$3.310B	$3.730B
YoY Change	12.19%	0.34%	0.73%
Goodwill	$6.227B	$6.219B
YoY Change	8.27%	8.18%
Intangibles	$8.285B	$8.363B
YoY Change	356.89%	352.03%
Long-Term Investments	$697.6M	$745.0M	$745.0M
YoY Change	-70.43%	-66.67%	-66.67%
Other Assets	$898.3M	$745.0M	$928.6M
YoY Change	-34.94%	-51.28%	-24.28%
Total Long-Term Assets	$19.81B	$19.99B	$19.99B
YoY Change	33.54%	35.38%	35.38%
TOTAL ASSETS
Total Short-Term Assets	$6.756B	$6.859B	$6.859B
Total Long-Term Assets	$19.81B	$19.99B	$19.99B
Total Assets	$26.57B	$26.84B	$26.84B
YoY Change	8.01%	9.33%	9.33%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$387.0M	$403.3M	$403.3M
YoY Change	-21.21%	-17.95%	-17.95%
Accrued Expenses	$1.362B	$2.624B	$2.624B
YoY Change	6.53%	75.3%	75.3%
Deferred Revenue
YoY Change
Short-Term Debt	$0.00	$0.00	$0.00
YoY Change
Long-Term Debt Due	$250.0M	$150.0M	$150.0M
YoY Change
Total Short-Term Liabilities	$3.223B	$3.434B	$3.434B
YoY Change	6.9%	4.93%	4.93%
LONG-TERM LIABILITIES
Long-Term Debt	$6.290B	$6.788B	$6.788B
YoY Change	0.12%	8.08%	8.08%
Other Long-Term Liabilities	$1.184B	$781.1M	$1.181B
YoY Change	26.52%	-17.27%	-7.52%
Total Long-Term Liabilities	$7.474B	$781.1M	$7.969B
YoY Change	698.9%	-17.27%	5.44%
TOTAL LIABILITIES
Total Short-Term Liabilities	$3.223B	$3.434B	$3.434B
Total Long-Term Liabilities	$7.474B	$781.1M	$7.969B
Total Liabilities	$11.35B	$12.05B	$12.05B
YoY Change	5.02%	7.88%	7.97%
SHAREHOLDERS EQUITY
Retained Earnings	$18.02B	$17.63B
YoY Change	6.92%	7.05%
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
Shareholders Equity	$15.21B	$14.80B	$14.80B
YoY Change
Total Liabilities & Shareholders Equity	$26.57B	$26.84B	$26.84B
YoY Change	8.01%	9.33%	9.33%

Cashflow Statement

Concept	2024 Q1	2023 Q4	2023
OPERATING ACTIVITIES
Net Income	$393.4M	$249.7M	$1.161B
YoY Change	1.42%	-54.63%	-61.89%
Depreciation, Depletion And Amortization	$147.5M	$76.60M	$494.8M
YoY Change	31.34%	-36.06%	-4.55%
Cash From Operating Activities	$553.2M	$12.50M	$1.547B
YoY Change	21.5%	-107.14%	11.77%
INVESTING ACTIVITIES
Capital Expenditures	$120.9M	$65.20M	$277.0M
YoY Change	81.53%	-25.4%	15.27%
Acquisitions			$6.926B
YoY Change
Other Investing Activities	$54.90M	-$587.1M	$11.30M
YoY Change	7742.86%	993.3%	-146.31%
Cash From Investing Activities	-$66.00M	-$652.3M	-$4.101B
YoY Change	-93.07%	362.3%	-360.12%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net			$0.00
YoY Change			-100.0%
Debt Paid & Issued, Net	$400.0M
YoY Change
Cash From Financing Activities	-439.6M	-646.1M	$149.3M
YoY Change	912.9%	8631.08%	-108.54%
NET CHANGE
Cash From Operating Activities	553.2M	12.50M	$1.547B
Cash From Investing Activities	-66.00M	-652.3M	-$4.101B
Cash From Financing Activities	-439.6M	-646.1M	$149.3M
Net Change In Cash	47.60M	-1.286B	-$2.405B
YoY Change	-108.8%	297.5%	-298.13%
FREE CASH FLOW
Cash From Operating Activities	$553.2M	$12.50M	$1.547B
Capital Expenditures	$120.9M	$65.20M	$277.0M
Free Cash Flow	$432.3M	-$52.70M	$1.270B
YoY Change	11.22%	-79.92%	11.03%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2023	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	0	usd
CY2023	biib	Income Loss From Equity Method Investments Net Of Tax IncomeLossFromEquityMethodInvestmentsNetOfTax	0	usd
CY2023	us-gaap	Profit Loss ProfitLoss	1161500000	usd
CY2022	us-gaap	Profit Loss ProfitLoss	2961600000	usd
CY2023	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	400000	usd
CY2022	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-85300000	usd
CY2021	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	171500000	usd
CY2023	us-gaap	Net Income Loss NetIncomeLoss	1161100000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	3046900000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	1556100000	usd
CY2022	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	20.96
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	10.44
CY2023	us-gaap	Inventory Write Down InventoryWriteDown	124400000	usd
CY2022	us-gaap	Inventory Write Down InventoryWriteDown	336200000	usd
CY2020Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	10686100000	usd
CY2023	biib	Interest In Subsidiary InterestInSubsidiary	1.000
CY2023	us-gaap	Use Of Estimates UseOfEstimates	<div style="margin-top:9pt"><span style="color:#0070c0;font-family:'Franklin Gothic Book',sans-serif;font-size:11pt;font-weight:700;line-height:120%">Use of Estimates</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%">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>
CY2023	dei	Entity Central Index Key EntityCentralIndexKey	0000875045
CY2023	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2023
CY2023	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2023	dei	Amendment Flag AmendmentFlag	false
CY2022	us-gaap	Impairment Of Intangible Asset Indefinite Lived Excluding Goodwill Statement Of Income Or Comprehensive Income Extensible Enumeration ImpairmentOfIntangibleAssetIndefiniteLivedExcludingGoodwillStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration	http://www.biogenidec.com/20231231#AmortizationandImpairmentofAcquiredIntangibleAssets
CY2023Q4	us-gaap	Operating Lease Right Of Use Asset Statement Of Financial Position Extensible List OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OperatingLeaseRightOfUseAsset
CY2022Q4	us-gaap	Operating Lease Right Of Use Asset Statement Of Financial Position Extensible List OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OperatingLeaseRightOfUseAsset
CY2022Q4	us-gaap	Operating Lease Liability Current Statement Of Financial Position Extensible List OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OtherLiabilitiesCurrent
CY2023Q4	us-gaap	Operating Lease Liability Current Statement Of Financial Position Extensible List OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OtherLiabilitiesCurrent
CY2022Q4	us-gaap	Operating Lease Liability Noncurrent Statement Of Financial Position Extensible List OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OperatingLeaseLiabilityNoncurrent
CY2023Q4	us-gaap	Operating Lease Liability Noncurrent Statement Of Financial Position Extensible List OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OperatingLeaseLiabilityNoncurrent
CY2023	dei	Document Annual Report DocumentAnnualReport	true
CY2023	dei	Document Period End Date DocumentPeriodEndDate	2023-12-31
CY2023	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2023	dei	Document Transition Report DocumentTransitionReport	false
CY2023	dei	Entity File Number EntityFileNumber	0-19311
CY2023	dei	Entity Registrant Name EntityRegistrantName	BIOGEN INC.
CY2023	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
CY2023	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	33-0112644
CY2023	dei	Entity Address Address Line1 EntityAddressAddressLine1	225 Binney Street
CY2023	dei	Entity Address City Or Town EntityAddressCityOrTown	Cambridge
CY2023	dei	Entity Address State Or Province EntityAddressStateOrProvince	MA
CY2023	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	02142
CY2023	dei	City Area Code CityAreaCode	617
CY2023	dei	Local Phone Number LocalPhoneNumber	679-2000
CY2023	dei	Security12b Title Security12bTitle	Common Stock, $0.0005 par value
CY2023	dei	Trading Symbol TradingSymbol	BIIB
CY2023	dei	Security Exchange Name SecurityExchangeName	NASDAQ
CY2023	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2023	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2023	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2023	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2023	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2023	dei	Entity Small Business EntitySmallBusiness	false
CY2023	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2023	dei	Icfr Auditor Attestation Flag IcfrAuditorAttestationFlag	true
CY2023	dei	Document Fin Stmt Error Correction Flag DocumentFinStmtErrorCorrectionFlag	false
CY2023	dei	Entity Shell Company EntityShellCompany	false
CY2023Q2	dei	Entity Public Float EntityPublicFloat	41190868800	usd
CY2023	ecd	Non Rule10b51 Arr Trmntd Flag NonRule10b51ArrTrmntdFlag	false
CY2023	ecd	Rule10b51 Arr Trmntd Flag Rule10b51ArrTrmntdFlag	false
CY2023	ecd	Non Rule10b51 Arr Adopted Flag NonRule10b51ArrAdoptedFlag	false
CY2023	dei	Auditor Firm AuditorFirmId	238
CY2023	us-gaap	Revenues Revenues	9835600000	usd
CY2022	us-gaap	Revenues Revenues	10173400000	usd
CY2021	us-gaap	Revenues Revenues	10981700000	usd
CY2023	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	2533400000	usd
CY2022	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	2278300000	usd
CY2021	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	2109700000	usd
CY2023	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2462000000	usd
CY2022	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2231100000	usd
CY2021	us-gaap	Research And Development Expense Excluding Acquired In Process Cost ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost	2501200000	usd
CY2023	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2549700000	usd
CY2022	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2403600000	usd
CY2021	us-gaap	Selling General And Administrative Expense SellingGeneralAndAdministrativeExpense	2674300000	usd
CY2023	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	240600000	usd
CY2022	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	365900000	usd
CY2021	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	881300000	usd
CY2023	biib	Collaboration Profit Loss Sharing CollaborationProfitLossSharing	218800000	usd
CY2022	biib	Collaboration Profit Loss Sharing CollaborationProfitLossSharing	-7400000	usd
CY2021	biib	Collaboration Profit Loss Sharing CollaborationProfitLossSharing	7200000	usd
CY2022	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	209100000	usd
CY2021	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	50700000	usd
CY2023	biib	Acquired In Process Research And Development AcquiredInProcessResearchAndDevelopment	0	usd
CY2022	biib	Acquired In Process Research And Development AcquiredInProcessResearchAndDevelopment	0	usd
CY2021	biib	Acquired In Process Research And Development AcquiredInProcessResearchAndDevelopment	18000000.0	usd
CY2023	us-gaap	Restructuring Charges RestructuringCharges	218800000	usd
CY2022	us-gaap	Restructuring Charges RestructuringCharges	131100000	usd
CY2021	us-gaap	Restructuring Charges RestructuringCharges	0	usd
CY2023	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	0	usd
CY2022	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	503700000	usd
CY2021	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	0	usd
CY2023	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-315500000	usd
CY2022	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	108200000	usd
CY2021	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-1095500000	usd
CY2023	us-gaap	Costs And Expenses CostsAndExpenses	8538800000	usd
CY2022	us-gaap	Costs And Expenses CostsAndExpenses	6581600000	usd
CY2021	us-gaap	Costs And Expenses CostsAndExpenses	9236500000	usd
CY2023	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	1296800000	usd
CY2022	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	3591800000	usd
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	1745200000	usd
CY2023	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	135300000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	632800000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	52500000	usd
CY2022	biib	Income Loss From Equity Method Investments Net Of Tax IncomeLossFromEquityMethodInvestmentsNetOfTax	2600000	usd
CY2021	biib	Income Loss From Equity Method Investments Net Of Tax IncomeLossFromEquityMethodInvestmentsNetOfTax	34900000	usd
CY2021	us-gaap	Profit Loss ProfitLoss	1727600000	usd
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	10.40
CY2023	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss After Reclassification And Tax Parent OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent	-40100000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss After Reclassification And Tax Parent OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent	-38700000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss After Reclassification And Tax Parent OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent	232800000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	0	usd
CY2022	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	-25500000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments And Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsAndTax	34000000.0	usd
CY2023	us-gaap	Comprehensive Income Net Of Tax Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest	400000	usd
CY2023	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	1172700000	usd
CY2022	us-gaap	Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest	2903400000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	1049900000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	3419300000	usd
CY2023Q4	us-gaap	Available For Sale Securities Debt Securities Current AvailableForSaleSecuritiesDebtSecuritiesCurrent	0	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Securities Current AvailableForSaleSecuritiesDebtSecuritiesCurrent	1473500000	usd
CY2023Q4	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	2400000	usd
CY2023Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	1664100000	usd
CY2022Q4	us-gaap	Accounts Receivable Net Current AccountsReceivableNetCurrent	1705000000	usd
CY2023Q4	biib	Due From Unconsolidated Joint Business DueFromUnconsolidatedJointBusiness	435900000	usd
CY2022Q4	biib	Due From Unconsolidated Joint Business DueFromUnconsolidatedJointBusiness	431400000	usd
CY2023Q4	us-gaap	Inventory Net InventoryNet	2527400000	usd
CY2023Q4	us-gaap	Other Assets Current OtherAssetsCurrent	1182000000	usd
CY2023Q4	us-gaap	Assets Current AssetsCurrent	6859300000	usd
CY2023Q4	us-gaap	Available For Sale Securities Debt Securities Noncurrent AvailableForSaleSecuritiesDebtSecuritiesNoncurrent	0	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Securities Noncurrent AvailableForSaleSecuritiesDebtSecuritiesNoncurrent	705700000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3298600000	usd
CY2022Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	403900000	usd
CY2022Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	1850100000	usd
CY2022Q4	us-gaap	Goodwill Goodwill	5749000000	usd
CY2022Q4	us-gaap	Deferred Income Tax Assets Net DeferredIncomeTaxAssetsNet	1226400000	usd
CY2022Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	1529200000	usd
CY2022Q4	us-gaap	Assets Assets	24554100000	usd
CY2023Q4	us-gaap	Debt Current DebtCurrent	150000000.0	usd
CY2022Q4	us-gaap	Debt Current DebtCurrent	0	usd
CY2022Q4	us-gaap	Taxes Payable Current TaxesPayableCurrent	259900000	usd
CY2022Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	491500000	usd
CY2022Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2521400000	usd
CY2022Q4	us-gaap	Liabilities Current LiabilitiesCurrent	3272800000	usd
CY2023Q4	us-gaap	Long Term Notes Payable LongTermNotesPayable	6788200000	usd
CY2022Q4	us-gaap	Long Term Notes Payable LongTermNotesPayable	6281000000	usd
CY2022Q4	us-gaap	Deferred Income Tax Liabilities Net DeferredIncomeTaxLiabilitiesNet	334700000	usd
CY2022Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	333000000.0	usd
CY2022Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	944200000	usd
CY2022Q4	us-gaap	Liabilities Liabilities	11165700000	usd
CY2022Q4	us-gaap	Commitments And Contingencies CommitmentsAndContingencies		usd
CY2023Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.001
CY2022Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.001
CY2023Q4	us-gaap	Preferred Stock Value Outstanding PreferredStockValueOutstanding	0	usd
CY2022Q4	us-gaap	Preferred Stock Value Outstanding PreferredStockValueOutstanding	0	usd
CY2022Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0005
CY2023Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0005
CY2023Q4	us-gaap	Common Stock Value Outstanding CommonStockValueOutstanding	100000	usd
CY2022Q4	us-gaap	Common Stock Value Outstanding CommonStockValueOutstanding	100000	usd
CY2023Q4	us-gaap	Additional Paid In Capital AdditionalPaidInCapital	302500000	usd
CY2023Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-153700000	usd
CY2023Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	17627600000	usd
CY2023Q4	us-gaap	Treasury Stock Common Shares TreasuryStockCommonShares	23800000	shares
CY2022Q4	us-gaap	Treasury Stock Common Shares TreasuryStockCommonShares	23800000	shares
CY2022Q4	us-gaap	Treasury Stock Value TreasuryStockValue	2977100000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	14799400000	usd
CY2022Q4	us-gaap	Stockholders Equity StockholdersEquity	13397900000	usd
CY2023Q4	us-gaap	Minority Interest MinorityInterest	0	usd
CY2022Q4	us-gaap	Minority Interest MinorityInterest	-9500000	usd
CY2023Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	14799400000	usd
CY2022Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13388400000	usd
CY2023Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	26844800000	usd
CY2022Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	24554100000	usd
CY2022	us-gaap	Profit Loss ProfitLoss	2961600000	usd
CY2021	us-gaap	Profit Loss ProfitLoss	1727600000	usd
CY2023	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	494800000	usd
CY2022	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	518400000	usd
CY2021	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	487700000	usd
CY2023	us-gaap	Impairment Of Intangible Assets Excluding Goodwill ImpairmentOfIntangibleAssetsExcludingGoodwill	0	usd
CY2022	us-gaap	Impairment Of Intangible Assets Excluding Goodwill ImpairmentOfIntangibleAssetsExcludingGoodwill	119600000	usd
CY2021	us-gaap	Impairment Of Intangible Assets Excluding Goodwill ImpairmentOfIntangibleAssetsExcludingGoodwill	629300000	usd
CY2021	us-gaap	Inventory Write Down InventoryWriteDown	167600000	usd
CY2023	biib	Amortization Of Inventory Step Up AmortizationOfInventoryStepUp	31500000	usd
CY2022	biib	Amortization Of Inventory Step Up AmortizationOfInventoryStepUp	0	usd
CY2021	biib	Amortization Of Inventory Step Up AmortizationOfInventoryStepUp	0	usd
CY2023	biib	Payments To Acquire In Process Research And Development Operating Activities PaymentsToAcquireInProcessResearchAndDevelopmentOperatingActivities	0	usd
CY2022	biib	Payments To Acquire In Process Research And Development Operating Activities PaymentsToAcquireInProcessResearchAndDevelopmentOperatingActivities	0	usd
CY2021	biib	Payments To Acquire In Process Research And Development Operating Activities PaymentsToAcquireInProcessResearchAndDevelopmentOperatingActivities	-18000000.0	usd
CY2023	us-gaap	Share Based Compensation ShareBasedCompensation	264200000	usd
CY2022	us-gaap	Share Based Compensation ShareBasedCompensation	254100000	usd
CY2021	us-gaap	Share Based Compensation ShareBasedCompensation	238600000	usd
CY2023	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	0	usd
CY2022	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	209100000	usd
CY2021	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	50700000	usd
CY2023	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-305800000	usd
CY2022	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-168600000	usd
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-426800000	usd
CY2023	us-gaap	Increase Decrease In Equity Securities Fv Ni IncreaseDecreaseInEquitySecuritiesFvNi	-277100000	usd
CY2022	us-gaap	Increase Decrease In Equity Securities Fv Ni IncreaseDecreaseInEquitySecuritiesFvNi	-265900000	usd
CY2021	us-gaap	Increase Decrease In Equity Securities Fv Ni IncreaseDecreaseInEquitySecuritiesFvNi	-826800000	usd
CY2023	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	0	usd
CY2022	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	2600000	usd
CY2021	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	34900000	usd
CY2023	us-gaap	Equity Method Investment Realized Gain Loss On Disposal EquityMethodInvestmentRealizedGainLossOnDisposal	0	usd
CY2022	us-gaap	Equity Method Investment Realized Gain Loss On Disposal EquityMethodInvestmentRealizedGainLossOnDisposal	1505400000	usd
CY2021	us-gaap	Equity Method Investment Realized Gain Loss On Disposal EquityMethodInvestmentRealizedGainLossOnDisposal	0	usd
CY2023	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	0	usd
CY2022	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	503700000	usd
CY2021	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	0	usd
CY2023	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-148200000	usd
CY2022	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-208200000	usd
CY2021	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-202200000	usd
CY2023	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-61300000	usd
CY2022	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	203400000	usd
CY2021	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-324800000	usd
CY2023	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	4600000	usd
CY2022	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	19000000.0	usd
CY2021	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-1200000	usd
CY2023	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	130900000	usd
CY2022	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	320200000	usd
CY2021	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	462400000	usd
CY2023	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-201600000	usd
CY2022	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-113400000	usd
CY2021	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-95400000	usd
CY2023	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	-299000000.0	usd
CY2022	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	-142300000	usd
CY2021	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	230800000	usd
CY2023	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-73900000	usd
CY2022	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-92000000.0	usd
CY2021	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-144500000	usd
CY2023	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	1547200000	usd
CY2022	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	1384300000	usd
CY2021	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	3639900000	usd
CY2023	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	277000000.0	usd
CY2022	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	240300000	usd
CY2021	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	258100000	usd
CY2023	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	7380800000	usd
CY2022	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	3671000000	usd
CY2021	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	3405400000	usd
CY2023	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	5140700000	usd
CY2022	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	3448500000	usd
CY2021	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	3808700000	usd
CY2023	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	6926100000	usd
CY2023	us-gaap	Proceeds From Divestiture Of Interest In Consolidated Subsidiaries ProceedsFromDivestitureOfInterestInConsolidatedSubsidiaries	788100000	usd
CY2022	us-gaap	Proceeds From Divestiture Of Interest In Consolidated Subsidiaries ProceedsFromDivestitureOfInterestInConsolidatedSubsidiaries	990300000	usd
CY2021	us-gaap	Proceeds From Divestiture Of Interest In Consolidated Subsidiaries ProceedsFromDivestitureOfInterestInConsolidatedSubsidiaries	0	usd
CY2023	us-gaap	Payments To Acquire Business Two Net Of Cash Acquired PaymentsToAcquireBusinessTwoNetOfCashAcquired	0	usd
CY2022	us-gaap	Payments To Acquire Business Two Net Of Cash Acquired PaymentsToAcquireBusinessTwoNetOfCashAcquired	-582600000	usd
CY2021	us-gaap	Payments To Acquire Business Two Net Of Cash Acquired PaymentsToAcquireBusinessTwoNetOfCashAcquired	0	usd
CY2023	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	usd
CY2022	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	usd
CY2021	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	28100000	usd
CY2021	us-gaap	Payments To Acquire In Process Research And Development PaymentsToAcquireInProcessResearchAndDevelopment	18000000.0	usd
CY2023	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	34400000	usd
CY2022	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	2900000	usd
CY2021	us-gaap	Payments To Acquire Intangible Assets PaymentsToAcquireIntangibleAssets	18800000	usd
CY2023	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	119600000	usd
CY2022	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	0	usd
CY2021	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	93500000	usd
CY2023	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	11300000	usd
CY2022	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-24400000	usd
CY2021	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-12900000	usd
CY2023	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-4101000000	usd
CY2022	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	1576600000	usd
CY2021	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-563700000	usd
CY2023	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	0	usd
CY2022	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	750000000.0	usd
CY2021	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	1800000000	usd
CY2023	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	44300000	usd
CY2022	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	1900000	usd
CY2021	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	700000	usd
CY2023	biib	Repayments Of Debt Including Premiums Paid RepaymentsOfDebtIncludingPremiumsPaid	809900000	usd
CY2022	biib	Repayments Of Debt Including Premiums Paid RepaymentsOfDebtIncludingPremiumsPaid	1002200000	usd
CY2021	biib	Repayments Of Debt Including Premiums Paid RepaymentsOfDebtIncludingPremiumsPaid	170000000.0	usd
CY2023	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	997200000	usd
CY2022	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	0	usd
CY2021	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	0	usd
CY2023	us-gaap	Proceeds From Payments To Minority Shareholders ProceedsFromPaymentsToMinorityShareholders	12300000	usd
CY2022	us-gaap	Proceeds From Payments To Minority Shareholders ProceedsFromPaymentsToMinorityShareholders	12400000	usd
CY2021	us-gaap	Proceeds From Payments To Minority Shareholders ProceedsFromPaymentsToMinorityShareholders	-94400000	usd
CY2023	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-6000000.0	usd
CY2022	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-5600000	usd
CY2021	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-21100000	usd
CY2023	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	149300000	usd
CY2022	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-1747300000	usd
CY2021	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-2086200000	usd
CY2023	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	-2404500000	usd
CY2022	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	1213600000	usd
CY2021	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	990000000.0	usd
CY2023	us-gaap	Effect Of Exchange Rate On Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	35100000	usd
CY2022	us-gaap	Effect Of Exchange Rate On Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	-55700000	usd
CY2021	us-gaap	Effect Of Exchange Rate On Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	-59800000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	3419300000	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
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	1049900000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	3419300000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2261400000	usd
CY2022Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13388400000	usd
CY2023	us-gaap	Profit Loss ProfitLoss	1161500000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	11200000	usd
CY2023	biib	Noncontrolling Interest Increase Decrease Other NoncontrollingInterestIncreaseDecreaseOther	12300000	usd
CY2023	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	3200000	usd
CY2023	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	45100000	usd
CY2023	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	-89500000	usd
CY2023	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	-274400000	usd
CY2023	us-gaap	Stockholders Equity Other StockholdersEquityOther	800000	usd
CY2023Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	14799400000	usd
CY2021Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	10959700000	usd
CY2022	us-gaap	Profit Loss ProfitLoss	2961600000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-58200000	usd
CY2022	biib	Noncontrolling Interest Increase Decrease Other NoncontrollingInterestIncreaseDecreaseOther	12300000	usd
CY2022	us-gaap	Stock Issued During Period Value Treasury Stock Reissued StockIssuedDuringPeriodValueTreasuryStockReissued	44200000	usd
CY2022	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	-46000000.0	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	-263500000	usd
CY2022	us-gaap	Stockholders Equity Other StockholdersEquityOther	-1300000	usd
CY2022Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	13388400000	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
CY2023	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	<div style="margin-top:9pt"><span style="color:#0070c0;font-family:'Franklin Gothic Book',sans-serif;font-size:11pt;font-weight:700;line-height:120%">Concentration of Credit Risk</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%">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.
CY2023	us-gaap	Number Of Operating Segments NumberOfOperatingSegments	1	segment
CY2023	us-gaap	Number Of Reportable Segments NumberOfReportableSegments	1	segment
CY2023	us-gaap	Advertising Expense AdvertisingExpense	71400000	usd
CY2022	us-gaap	Advertising Expense AdvertisingExpense	54100000	usd
CY2021	us-gaap	Advertising Expense AdvertisingExpense	98700000	usd
CY2023	biib	Number Of Wholesalers NumberOfWholesalers	2	wholesaler
CY2023	biib	Number Of Wholesalers NumberOfWholesalers	2	wholesaler
CY2022Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1035000000	usd
CY2023	biib	Current Provisions To Product Revenue Reserves Relating To Sales In Current Year CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear	3474700000	usd
CY2023	biib	Adjustments To Product Revenue Reserves Relating To Sales In Prior Years AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears	19600000	usd
CY2023	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Current Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear	2519800000	usd
CY2023	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear	908300000	usd
CY2023Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1062000000	usd
CY2021Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	935300000	usd
CY2022	biib	Current Provisions To Product Revenue Reserves Relating To Sales In Current Year CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear	3394400000	usd
CY2022	biib	Adjustments To Product Revenue Reserves Relating To Sales In Prior Years AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears	8600000	usd
CY2022	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Current Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear	2576800000	usd
CY2022	biib	Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear	709300000	usd
CY2022Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1035000000	usd
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
CY2023Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1062000000	usd
CY2022Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	1035000000	usd
CY2023	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.172
CY2022	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.167
CY2021	biib	Revenues In Percentage From Anti Cd20 Therapeutic Programs RevenuesInPercentageFromAntiCD20TherapeuticPrograms	0.151
CY2023Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Five FiniteLivedIntangibleAssetsAmortizationExpenseYearFive	495000000.0	usd
CY2023Q4	us-gaap	Inventory Raw Materials Net Of Reserves InventoryRawMaterialsNetOfReserves	426900000	usd
CY2022Q4	us-gaap	Inventory Raw Materials Net Of Reserves InventoryRawMaterialsNetOfReserves	413200000	usd
CY2023Q4	us-gaap	Inventory Work In Process Net Of Reserves InventoryWorkInProcessNetOfReserves	1926800000	usd
CY2022Q4	us-gaap	Inventory Work In Process Net Of Reserves InventoryWorkInProcessNetOfReserves	751900000	usd
CY2023Q4	us-gaap	Inventory Finished Goods Net Of Reserves InventoryFinishedGoodsNetOfReserves	255400000	usd
CY2022Q4	us-gaap	Inventory Finished Goods Net Of Reserves InventoryFinishedGoodsNetOfReserves	200400000	usd
CY2023Q4	biib	Inventory Net Current And Noncurrent InventoryNetCurrentAndNoncurrent	2609100000	usd
CY2022Q4	biib	Inventory Net Current And Noncurrent InventoryNetCurrentAndNoncurrent	1365500000	usd
CY2023Q4	us-gaap	Inventory Net InventoryNet	2527400000	usd
CY2022Q4	us-gaap	Inventory Net InventoryNet	1344400000	usd
CY2023Q4	us-gaap	Inventory Noncurrent InventoryNoncurrent	81700000	usd
CY2022Q4	us-gaap	Inventory Noncurrent InventoryNoncurrent	21100000	usd
CY2023Q4	biib	Inventory Net Current And Noncurrent InventoryNetCurrentAndNoncurrent	2609100000	usd
CY2022Q4	biib	Inventory Net Current And Noncurrent InventoryNetCurrentAndNoncurrent	1365500000	usd
CY2023	us-gaap	Inventory Write Down InventoryWriteDown	124400000	usd
CY2022	us-gaap	Inventory Write Down InventoryWriteDown	336200000	usd
CY2021	us-gaap	Inventory Write Down InventoryWriteDown	167600000	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	11728800000	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5869800000	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	5859000000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	7415300000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5629200000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	1786100000	usd
CY2023Q4	us-gaap	Intangible Assets Gross Excluding Goodwill IntangibleAssetsGrossExcludingGoodwill	14232800000	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5869800000	usd
CY2023Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	8363000000	usd
CY2022Q4	us-gaap	Intangible Assets Gross Excluding Goodwill IntangibleAssetsGrossExcludingGoodwill	7479300000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	5629200000	usd
CY2022Q4	us-gaap	Intangible Assets Net Excluding Goodwill IntangibleAssetsNetExcludingGoodwill	1850100000	usd
CY2023	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	240600000	usd
CY2022	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	365900000	usd
CY2021	biib	Amortizationand Impairmentof Acquired Intangible Assets AmortizationandImpairmentofAcquiredIntangibleAssets	881300000	usd
CY2023	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	240600000	usd
CY2022	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	246300000	usd
CY2021	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	252000000	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Next Twelve Months FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths	345000000.0	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Two FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo	470000000.0	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Three FiniteLivedIntangibleAssetsAmortizationExpenseYearThree	485000000.0	usd
CY2023Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Four FiniteLivedIntangibleAssetsAmortizationExpenseYearFour	480000000.0	usd
CY2022Q4	us-gaap	Goodwill Goodwill	5749000000	usd
CY2021Q4	us-gaap	Goodwill Goodwill	5761100000	usd
CY2023	us-gaap	Goodwill Acquired During Period GoodwillAcquiredDuringPeriod	464500000	usd
CY2022	us-gaap	Goodwill Acquired During Period GoodwillAcquiredDuringPeriod	0	usd
CY2023	us-gaap	Goodwill Other Increase Decrease GoodwillOtherIncreaseDecrease	5700000	usd
CY2022	us-gaap	Goodwill Other Increase Decrease GoodwillOtherIncreaseDecrease	-12100000	usd
CY2023Q4	us-gaap	Goodwill Goodwill	6219200000	usd
CY2022Q4	us-gaap	Goodwill Goodwill	5749000000	usd
CY2023Q4	us-gaap	Goodwill Impaired Accumulated Impairment Loss GoodwillImpairedAccumulatedImpairmentLoss	0	usd
CY2022	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2023	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2022	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Gain Loss Included In Earnings FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings	209100000	usd
CY2023Q4	us-gaap	Cash Equivalents At Carrying Value CashEquivalentsAtCarryingValue	610700000	usd
CY2022Q4	us-gaap	Cash Equivalents At Carrying Value CashEquivalentsAtCarryingValue	2847600000	usd
CY2023Q4	us-gaap	Equity Securities Fv Ni Cost EquitySecuritiesFvNiCost	979900000	usd
CY2023Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Gain Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTax	0	usd
CY2023Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Loss Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTax	563100000	usd
CY2023Q4	us-gaap	Equity Securities Fv Ni EquitySecuritiesFvNi	416800000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2199100000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	300000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	20200000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2179200000	usd
CY2022Q4	us-gaap	Equity Securities Fv Ni Cost EquitySecuritiesFvNiCost	1133800000	usd
CY2022Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Gain Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTax	0	usd
CY2022Q4	biib	Equity Securities Available For Sale Accumulated Gross Unrealized Loss Before Tax EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTax	342700000	usd
CY2022Q4	us-gaap	Equity Securities Fv Ni EquitySecuritiesFvNi	791100000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Fair Value AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue	1473500000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost	1483300000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	694400000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost	703700000	usd
CY2022Q4	biib	Available For Sale Securities Debt Maturities After Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsFairValue	11300000	usd
CY2022Q4	biib	Available For Sale Securities Debt Maturities After Five Years Amortized Cost AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsAmortizedCost	12100000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	2179200000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	2199100000	usd
CY2022	biib	Average Maturity Of Marketable Securities AverageMaturityOfMarketableSecurities	P8M
CY2023	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	7380800000	usd
CY2022	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	3671000000	usd
CY2021	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	3405400000	usd
CY2023	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	1400000	usd
CY2022	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	0	usd
CY2021	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	200000	usd
CY2023	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	18400000	usd
CY2022	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	12600000	usd
CY2021	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	4000000.0	usd
CY2023Q4	us-gaap	Derivative Remaining Maturity1 DerivativeRemainingMaturity1	P12M
CY2023	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	3800000	usd
CY2022	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	-34700000	usd
CY2021	us-gaap	Gain Loss On Foreign Currency Derivative Instruments Not Designated As Hedging Instruments GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments	-43300000	usd
CY2023Q4	us-gaap	Land Land	202400000	usd
CY2022Q4	us-gaap	Land Land	202400000	usd
CY2023Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	1601300000	usd
CY2022Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	1592900000	usd
CY2023Q4	us-gaap	Leasehold Improvements Gross LeaseholdImprovementsGross	135700000	usd
CY2022Q4	us-gaap	Leasehold Improvements Gross LeaseholdImprovementsGross	107700000	usd
CY2023Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	1703800000	usd
CY2022Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	1611500000	usd
CY2023Q4	biib	Computer Software And Hardware ComputerSoftwareAndHardware	1032100000	usd
CY2022Q4	biib	Computer Software And Hardware ComputerSoftwareAndHardware	999900000	usd
CY2023Q4	us-gaap	Furniture And Fixtures Gross FurnitureAndFixturesGross	61500000	usd
CY2022Q4	us-gaap	Furniture And Fixtures Gross FurnitureAndFixturesGross	61100000	usd
CY2023Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	975400000	usd
CY2022Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	888800000	usd
CY2023Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	5712200000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	5464300000	usd
CY2023Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	2402500000	usd
CY2022Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	2165700000	usd
CY2023Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3309700000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3298600000	usd
CY2023	us-gaap	Depreciation Depreciation	254200000	usd
CY2022	us-gaap	Depreciation Depreciation	272400000	usd
CY2021	us-gaap	Depreciation Depreciation	235300000	usd
CY2023	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	21700000	usd
CY2022	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	17100000	usd
CY2021	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	36300000	usd
CY2023Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	420000000.0	usd
CY2022Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	403900000	usd
CY2023Q4	us-gaap	Operating Lease Liability Current OperatingLeaseLiabilityCurrent	90300000	usd
CY2022Q4	us-gaap	Operating Lease Liability Current OperatingLeaseLiabilityCurrent	97200000	usd
CY2023Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	400000000.0	usd
CY2022Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	333000000.0	usd
CY2023Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	490300000	usd
CY2022Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	430200000	usd
CY2023	us-gaap	Lease Cost LeaseCost	140300000	usd
CY2022	us-gaap	Lease Cost LeaseCost	95600000	usd
CY2021	us-gaap	Lease Cost LeaseCost	97900000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	106200000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	96100000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	83500000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	86100000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Five LesseeOperatingLeaseLiabilityPaymentsDueYearFive	49400000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due After Year Five LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive	170400000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	591700000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	101400000	usd
CY2023Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	490300000	usd
CY2023Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P7Y4M13D
CY2022Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P4Y7M20D
CY2023Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.045
CY2022Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.037
CY2023	us-gaap	Operating Lease Payments OperatingLeasePayments	116400000	usd
CY2022	us-gaap	Operating Lease Payments OperatingLeasePayments	107400000	usd
CY2021	us-gaap	Operating Lease Payments OperatingLeasePayments	105800000	usd
CY2023	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	146000000.0	usd
CY2022	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	108300000	usd
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	18100000	usd
CY2022Q3	biib	Operating Lease Right Of Use Asset Derecognized OperatingLeaseRightOfUseAssetDerecognized	47400000	usd
CY2022Q3	biib	Operating Lease Liability Derecognized OperatingLeaseLiabilityDerecognized	52700000	usd
CY2022Q3	biib	Gain Loss On Restructuring Charges GainLossOnRestructuringCharges	5300000	usd
CY2015Q3	us-gaap	Debt Instrument Face Amount DebtInstrumentFaceAmount	1000000000	usd
CY2023Q4	us-gaap	Debt Instrument Interest Rate Stated Percentage DebtInstrumentInterestRateStatedPercentage	0.03625
CY2022Q3	biib	Debt Instrument Redemption Rate DebtInstrumentRedemptionRate	0.99920
CY2023Q4	us-gaap	Debt Instrument Interest Rate Stated Percentage DebtInstrumentInterestRateStatedPercentage	0.03625
CY2023Q4	us-gaap	Debt Instrument Interest Rate Stated Percentage DebtInstrumentInterestRateStatedPercentage	0.03625
CY2022Q3	us-gaap	Extinguishment Of Debt Amount ExtinguishmentOfDebtAmount	2400000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	150000000.0	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	1750000000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	500000000.0	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	0	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive	0	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal After Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive	4817300000	usd
CY2023Q4	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	7217300000	usd
CY2023Q4	us-gaap	Debt Instrument Unamortized Discount Premium And Debt Issuance Costs Net DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet	279100000	usd
CY2023Q4	us-gaap	Long Term Debt LongTermDebt	6938200000	usd
CY2023Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	8000000	shares
CY2021Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2023Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2022Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2023Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2022Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2021Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2023Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000	shares
CY2023Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	168700000	shares
CY2023Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	144900000	shares
CY2022Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	1000000000	shares
CY2022Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	167900000	shares
CY2022Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	144000000.0	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
CY2023	us-gaap	Treasury Stock Shares Acquired TreasuryStockSharesAcquired	0	shares
CY2021	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8000000.0	usd
CY2023	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	658200000	usd
CY2022	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	264300000	usd
CY2021	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	250800000	usd
CY2023Q4	us-gaap	Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized	287400000	usd
CY2023	biib	Weighted Average Period To Recognize Cost Of Nonvested Awards WeightedAveragePeriodToRecognizeCostOfNonvestedAwards	P1Y9M18D
CY2023	us-gaap	Net Income Loss NetIncomeLoss	1161100000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	3046900000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	1556100000	usd
CY2023	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	144700000	shares
CY2022	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	145300000	shares
CY2021	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	149100000	shares
CY2023	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	900000	shares
CY2022	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	700000	shares
CY2021	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	500000	shares
CY2023	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	145600000	shares
CY2022	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	146000000.0	shares
CY2021	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	149600000	shares
CY2023	us-gaap	Treasury Stock Shares Acquired TreasuryStockSharesAcquired	0	shares
CY2023	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	668400000	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	273600000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	258800000	usd
CY2023	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	10200000	usd
CY2022	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	9300000	usd
CY2021	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	8000000.0	usd
CY2023	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	658200000	usd
CY2022	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	264300000	usd
CY2021	biib	Share Based Compensation Expense Included In Costs And Expenses ShareBasedCompensationExpenseIncludedInCostsAndExpenses	250800000	usd
CY2023	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	132600000	usd
CY2022	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	49200000	usd
CY2021	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	46700000	usd
CY2023	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	668400000	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	273600000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	258800000	usd
CY2023	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	10200000	usd
CY2022	us-gaap	Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount	9300000	usd
CY2023	biib	Number Of Share Based Compensation Plans Pursuant To Which Awards Are Currently Being Made NumberOfShareBasedCompensationPlansPursuantToWhichAwardsAreCurrentlyBeingMade	3	plan
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	81000000	shares
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	301.85
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P9Y10M24D
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross	0	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	0
CY2023	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	0	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	0
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod	0	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Forfeitures In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice	0
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	81000000	shares
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	301.85
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P8Y10M24D
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	27000000	shares
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	301.85
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1	P8Y10M24D
CY2015Q2	us-gaap	Stock Issued During Period Shares Employee Stock Purchase Plans StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans	6200000	shares
CY2023	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	192400000	usd
CY2022	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	1842000000	usd
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	448300000	usd
CY2023	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	1104400000	usd
CY2022	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	1749800000	usd
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	1296900000	usd
CY2023	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	1296800000	usd
CY2022	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	3591800000	usd
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	1745200000	usd
CY2023	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	377600000	usd
CY2022	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	694500000	usd
CY2021	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	319100000	usd
CY2023	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	15100000	usd
CY2022	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	39000000.0	usd
CY2021	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	23100000	usd
CY2023	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	48400000	usd
CY2022	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	67900000	usd
CY2021	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	137100000	usd
CY2023	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	441100000	usd
CY2022	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	801400000	usd
CY2021	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	479300000	usd
CY2023	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-587400000	usd
CY2022	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-328300000	usd
CY2021	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-242500000	usd
CY2023	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-12700000	usd
CY2022	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	2500000	usd
CY2021	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-11900000	usd
CY2023	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	294300000	usd
CY2022	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	157200000	usd
CY2021	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-172400000	usd
CY2023	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-305800000	usd
CY2022	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-168600000	usd
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-426800000	usd
CY2023	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	135300000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	632800000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	52500000	usd
CY2023	us-gaap	Income Taxes Paid IncomeTaxesPaid	185400000	usd
CY2023Q4	biib	Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries	1500000000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	252800000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	112600000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Inventory DeferredTaxAssetsInventory	203700000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Inventory DeferredTaxAssetsInventory	202800000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Goodwill And Intangible Assets DeferredTaxAssetsGoodwillAndIntangibleAssets	1153900000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Goodwill And Intangible Assets DeferredTaxAssetsGoodwillAndIntangibleAssets	1370300000	usd
CY2023Q4	biib	Deferred Tax Assets Counterparty Tax Basis In Asset DeferredTaxAssetsCounterpartyTaxBasisInAsset	0	usd
CY2022Q4	biib	Deferred Tax Assets Counterparty Tax Basis In Asset DeferredTaxAssetsCounterpartyTaxBasisInAsset	470300000	usd
CY2023Q4	us-gaap	Deferred Tax Assets In Process Research And Development DeferredTaxAssetsInProcessResearchAndDevelopment	570800000	usd
CY2022Q4	us-gaap	Deferred Tax Assets In Process Research And Development DeferredTaxAssetsInProcessResearchAndDevelopment	271800000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	1700400000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	1845900000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	36100000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	37200000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	293300000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	280700000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	1278700000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	2003300000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2932300000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2588300000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets	1257400000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets	76100000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities Investment In Noncontrolled Affiliates DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates	35500000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Investment In Noncontrolled Affiliates DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates	138000000.0	usd
CY2023Q4	biib	Deferredtaxliabilities Gilt Itaxcalculation DeferredtaxliabilitiesGILTItaxcalculation	1136900000	usd
CY2022Q4	biib	Deferredtaxliabilities Gilt Itaxcalculation DeferredtaxliabilitiesGILTItaxcalculation	1002000000	usd
CY2023Q4	biib	Deferred Tax Liabilities Tax Credit Carryforwards DeferredTaxLiabilitiesTaxCreditCarryforwards	0	usd
CY2022Q4	biib	Deferred Tax Liabilities Tax Credit Carryforwards DeferredTaxLiabilitiesTaxCreditCarryforwards	228700000	usd
CY2023Q4	biib	Deferred Tax Liabilities Depreciation Amortization Other DeferredTaxLiabilitiesDepreciationAmortizationOther	215700000	usd
CY2022Q4	biib	Deferred Tax Liabilities Depreciation Amortization Other DeferredTaxLiabilitiesDepreciationAmortizationOther	251800000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	2645500000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	1696600000	usd
CY2023Q4	biib	Totaldeferredchargesandprepaidtaxes Totaldeferredchargesandprepaidtaxes	69300000	usd
CY2022Q4	biib	Totaldeferredchargesandprepaidtaxes Totaldeferredchargesandprepaidtaxes	56600000	usd
CY2023	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2022	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2021	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2023	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.011
CY2022	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.011
CY2021	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.008
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.059
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.049
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Foreign Income Tax Rate Differential EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	-0.105
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.073
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.017
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.038
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	0.007
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	0.003
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Depreciation And Amortization EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	-0.016
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Gilti Percent EffectiveIncomeTaxRateReconciliationGiltiPercent	-0.006
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Gilti Percent EffectiveIncomeTaxRateReconciliationGiltiPercent	0.007
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Gilti Percent EffectiveIncomeTaxRateReconciliationGiltiPercent	0.013
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Business EffectiveIncomeTaxRateReconciliationDispositionOfBusiness	0
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Business EffectiveIncomeTaxRateReconciliationDispositionOfBusiness	-0.016
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Disposition Of Business EffectiveIncomeTaxRateReconciliationDispositionOfBusiness	0
CY2023	biib	Effective Income Tax Rate Reconciliation Litigation Settlement Percent EffectiveIncomeTaxRateReconciliationLitigationSettlementPercent	0
CY2022	biib	Effective Income Tax Rate Reconciliation Litigation Settlement Percent EffectiveIncomeTaxRateReconciliationLitigationSettlementPercent	0.026
CY2021	biib	Effective Income Tax Rate Reconciliation Litigation Settlement Percent EffectiveIncomeTaxRateReconciliationLitigationSettlementPercent	0
CY2023	biib	Effective Income Tax Rate Reconciliation Deconsolidation Percent EffectiveIncomeTaxRateReconciliationDeconsolidationPercent	0
CY2022	biib	Effective Income Tax Rate Reconciliation Deconsolidation Percent EffectiveIncomeTaxRateReconciliationDeconsolidationPercent	0.023
CY2021	biib	Effective Income Tax Rate Reconciliation Deconsolidation Percent EffectiveIncomeTaxRateReconciliationDeconsolidationPercent	-0.053
CY2023	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	-0.001
CY2022	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	-0.014
CY2021	biib	Effective Income Tax Rate Reconciliation Intellectual Property Rights Reorganization EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization	0
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0.015
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	-0.008
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0.011
CY2023	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.104
CY2022	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.176
CY2021	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0.030
CY2022Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	100000000	usd
CY2023Q1	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	85000000	usd
CY2023	us-gaap	Equity Method Investment Realized Gain Loss On Disposal EquityMethodInvestmentRealizedGainLossOnDisposal	0	usd
CY2022	us-gaap	Equity Method Investment Realized Gain Loss On Disposal EquityMethodInvestmentRealizedGainLossOnDisposal	1505400000	usd
CY2021	us-gaap	Equity Method Investment Realized Gain Loss On Disposal EquityMethodInvestmentRealizedGainLossOnDisposal	0	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	606400000	usd
CY2021Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	563400000	usd
CY2020Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	75700000	usd
CY2023	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	5200000	usd
CY2022	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	36300000	usd
CY2021	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	4200000	usd
CY2023	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	60200000	usd
CY2022	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	23400000	usd
CY2021	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	509900000	usd
CY2023	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	485000000.0	usd
CY2022	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	14900000	usd
CY2021	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	18800000	usd
CY2023	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	2100000	usd
CY2022	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	1600000	usd
CY2021	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	3200000	usd
CY2023	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	11300000	usd
CY2022	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	200000	usd
CY2021	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	4400000	usd
CY2023Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	173400000	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	606400000	usd
CY2021Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	563400000	usd
CY2023Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	147600000	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	134000000	usd
CY2021Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	87500000	usd
CY2023	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	5100000	usd
CY2022	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	700000	usd
CY2021	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Expense UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense	2700000	usd
CY2023Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	30200000	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits Interest On Income Taxes Accrued UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	25200000	usd
CY2023Q4	us-gaap	Decrease In Unrecognized Tax Benefits Is Reasonably Possible DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible	20000000	usd
CY2023	us-gaap	Interest Paid Net InterestPaidNet	252200000	usd
CY2022	us-gaap	Interest Paid Net InterestPaidNet	262500000	usd
CY2021	us-gaap	Interest Paid Net InterestPaidNet	280800000	usd
CY2023	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	740700000	usd
CY2022	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	932900000	usd
CY2021	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	247900000	usd
CY2023	us-gaap	Litigation Settlement Expense LitigationSettlementExpense	0	usd
CY2022	us-gaap	Litigation Settlement Expense LitigationSettlementExpense	917000000.0	usd
CY2021	us-gaap	Litigation Settlement Expense LitigationSettlementExpense	0	usd
CY2023	us-gaap	Investment Income Interest InvestmentIncomeInterest	276500000	usd
CY2022	us-gaap	Investment Income Interest InvestmentIncomeInterest	89300000	usd
CY2021	us-gaap	Investment Income Interest InvestmentIncomeInterest	11000000.0	usd
CY2023	us-gaap	Interest Expense InterestExpense	246900000	usd
CY2022	us-gaap	Interest Expense InterestExpense	246600000	usd
CY2021	us-gaap	Interest Expense InterestExpense	253600000	usd
CY2023	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	291200000	usd
CY2022	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	277300000	usd
CY2021	us-gaap	Gain Loss On Sale Of Investments GainLossOnSaleOfInvestments	824900000	usd
CY2023	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	50400000	usd
CY2022	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	35500000	usd
CY2021	us-gaap	Foreign Currency Transaction Gain Loss Before Tax ForeignCurrencyTransactionGainLossBeforeTax	22400000	usd
CY2023	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	3500000	usd
CY2022	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	10100000	usd
CY2021	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	5600000	usd
CY2023	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-315500000	usd
CY2022	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	108200000	usd
CY2021	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-1095500000	usd
CY2022Q2	us-gaap	Litigation Settlement Expense LitigationSettlementExpense	900000000	usd
CY2023	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	275200000	usd
CY2022	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	264700000	usd
CY2021	us-gaap	Equity Securities Fv Ni Gain Loss EquitySecuritiesFvNiGainLoss	821100000	usd
CY2023	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	5200000	usd
CY2022	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	0	usd
CY2021	us-gaap	Equity Securities Fv Ni Realized Gain Loss EquitySecuritiesFvNiRealizedGainLoss	-10300000	usd
CY2023	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	270000000.0	usd
CY2022	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	264700000	usd
CY2021	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	831400000	usd
CY2023Q4	us-gaap	Settlement Liabilities Current SettlementLiabilitiesCurrent	117500000	usd
CY2022Q4	us-gaap	Settlement Liabilities Current SettlementLiabilitiesCurrent	0	usd
CY2023Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2623600000	usd
CY2022Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	2521400000	usd
CY2023Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	781100000	usd
CY2022Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	944200000	usd
CY2023Q4	us-gaap	Accrued Income Taxes Noncurrent AccruedIncomeTaxesNoncurrent	403200000	usd
CY2022Q4	us-gaap	Accrued Income Taxes Noncurrent AccruedIncomeTaxesNoncurrent	541700000	usd
CY2023Q4	biib	Share Of Net Profit From Sage Therapeutics Percent ShareOfNetProfitFromSageTherapeuticsPercent	0.500
CY2020	biib	Number Of Ophthalmolgy Biosimilar Products Exclusive Rights NumberOfOphthalmolgyBiosimilarProductsExclusiveRights	2	product
CY2022Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	100000000	usd
CY2023Q1	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	85000000	usd
CY2023Q4	biib	Investment In Biotechnology Companies That Are Determined To Be Unconsolidated Variable Interest Entities InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities	16400000	usd
CY2022Q4	biib	Investment In Biotechnology Companies That Are Determined To Be Unconsolidated Variable Interest Entities InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities	27800000	usd
CY2019Q4	us-gaap	Loss Contingency Estimate Of Possible Loss LossContingencyEstimateOfPossibleLoss	200000000	usd
CY2023Q2	us-gaap	Loss Contingency Estimate Of Possible Loss LossContingencyEstimateOfPossibleLoss	49900000	usd
CY2023Q4	biib	Potential Future Milestone Payments Commitment To Third Party Approximately PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately	5100000000	usd
CY2023Q4	biib	Accrued Expenses On Funding Commitment Related To Clinical Research AccruedExpensesOnFundingCommitmentRelatedToClinicalResearch	47200000	usd
CY2023Q4	us-gaap	Other Commitment OtherCommitment	669000000	usd
CY2023Q4	us-gaap	Liability For Uncertain Tax Positions Current LiabilityForUncertainTaxPositionsCurrent	172000000	usd
CY2023	us-gaap	Income Taxes Paid IncomeTaxesPaid	185400000	usd
CY2023	us-gaap	Number Of Reportable Segments NumberOfReportableSegments	1	segment
CY2023Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3729700000	usd
CY2022Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3702500000	usd
CY2021Q4	us-gaap	Noncurrent Assets NoncurrentAssets	3791800000	usd
CY2023	dei	Auditor Name AuditorName	PricewaterhouseCoopers LLP
CY2023	dei	Auditor Location AuditorLocation	Boston, Massachusetts

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$BIIB
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2024 Q1 Form 10-K Financial Statement

#000087504524000009 Filed on February 14, 2024

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$BIIB BIOGEN INC. Stock $biib

2024 Q1 Form 10-K Financial Statement

#000087504524000009 Filed on February 14, 2024

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$BIIB
BIOGEN INC. Stock $biib