Debt Instrument, Make Whole Provision Redemption Price, Percentage

No definition available.

Name:	biib_DebtInstrumentMakeWholeProvisionRedemptionPricePercentage
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Redemption Rate

+ References

No definition available.

+ Details

Name:	biib_DebtInstrumentRedemptionRate
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage Par Value Of Senior Notes

+ References

No definition available.

+ Details

Name:	biib_PercentageParValueOfSeniorNotes
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Pre-tax Charge, Payment of Early Call Premium and Write off of Remaining Unamortized Debt Issuance Costs, Net of Derivative Gain

+ References

No definition available.

+ Details

Name:	biib_PreTaxChargePaymentOfEarlyCallPremiumAndWriteOffOfRemainingUnamortizedDebtIssuanceCostsNetOfDerivativeGain
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=124435984&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124429444&loc=SL124452920-239629

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI https://asc.fasb.org/extlink&oid=123599511&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959

+ Details

Name:	us-gaap_GainLossOnDerivativeInstrumentsNetPretax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow for loan and debt issuance costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsOfFinancingCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_A2015SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_A2020SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_A225SeniorNotesDueMay12030Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_A315SeniorNotesDueMay12050Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Indebtedness - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	May 31, 2020	Jan. 31, 2020	Dec. 31, 2022	Dec. 31, 2020	Sep. 15, 2022	Feb. 28, 2021	Apr. 30, 2020	Sep. 15, 2015
Debt Instrument
Senior notes aggregate principal amount								$ 1,000,000,000
Interest rate on senior notes			3.625%
Redemption rate					99.92%
Revolving Credit Facility
Debt Instrument
Senior unsecured revolving credit facility maximum borrowing capacity		$ 1,000,000,000
Debt instrument, term		5 years
Outstanding borrowings			$ 0
Interest Expense \| Interest rate swap
Debt Instrument
Gain (loss) on derivative instruments, net, pretax	$ 3,300,000
2015 Senior Notes
Debt Instrument
Redemptions percentage of 2015 notes			100.00%
Redemption percentage for change in control provision			101.00%
Senior Notes \| Interest Expense \| Interest rate swap
Debt Instrument
Gain (loss) on derivative instruments, net, pretax				$ 3,300,000
Senior Notes \| 2020 Senior Notes
Debt Instrument
Senior notes aggregate principal amount							$ 3,000,000,000
Redemptions percentage of 2015 notes			100.00%
Redemption percentage for change in control provision			101.00%
Costs associated with the senior notes offerings			$ 24,400,000
Senior Notes \| 2.250% Senior Notes due May 1, 2030
Debt Instrument
Senior notes aggregate principal amount							$ 1,500,000,000
Interest rate on senior notes			2.25%				2.25%
Redemption percentage par value of senior notes							99.973%
Senior Notes \| 3.150% Senior Notes due May 1, 2050
Debt Instrument
Senior notes aggregate principal amount							$ 1,500,000,000
Interest rate on senior notes			3.15%				3.15%
Redemption percentage par value of senior notes							99.174%
Senior Notes \| 2015 Senior Notes
Debt Instrument
Costs associated with the senior notes offerings			$ 47,500,000
Senior Notes \| 3.625% Senior Notes due September 15, 2022(1)
Debt Instrument
Interest rate on senior notes			3.625%
Senior Notes \| 4.050% Senior Notes due September 15, 2025
Debt Instrument
Senior notes aggregate principal amount			$ 1,750,000,000
Interest rate on senior notes			4.05%
Redemption percentage par value of senior notes			99.764%
Senior Notes \| 5.200% Senior Notes due September 15, 2045
Debt Instrument
Senior notes aggregate principal amount			$ 1,120,000,000
Interest rate on senior notes			5.20%			5.20%
Redemption percentage par value of senior notes			99.294%
Senior Notes \| 2.900% Senior Notes due September 15, 2020
Debt Instrument
Senior notes aggregate principal amount								$ 1,500,000,000
Interest rate on senior notes	2.90%
Redemption percentage par value of senior notes								99.792%
Pre-tax charge				$ 12,700,000
Senior Notes \| 2.900% Senior Notes due September 15, 2020 \| Interest Expense
Debt Instrument
Pre-tax charge	$ 9,400,000