Debt Instrument, Make Whole Provision Redemption Price, Percentage

No definition available.

Name:	biib_DebtInstrumentMakeWholeProvisionRedemptionPricePercentage
Namespace Prefix:	biib_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Early Call Premium and Write off of Remaining Unamortized Debt Issuance Costs

+ References

No definition available.

+ Details

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

- Definition

Percentage Par Value Of Senior Notes

+ References

No definition available.

+ Details

Name:	biib_PercentageParValueOfSeniorNotes
Namespace Prefix:	biib_
Data Type:	num: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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ 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

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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num: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 http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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 http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

+ Details

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

- 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 http://asc.fasb.org/extlink&oid=118932676&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_DebtInstrumentAxis=biib_A2.25SeniorNotesdueMay12030Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=biib_A3.15SeniorNotesdueMay12050Member
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_A2.90SeniorNotesDueSept152020Member
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:

Indebtedness (Details) - USD ($)	6 Months Ended
Indebtedness (Details) - USD ($)	Jun. 30, 2020	Jun. 30, 2019	Apr. 30, 2020	Sep. 15, 2015
Debt Instrument [Line Items]
Payments of financing costs	$ 24,400,000
Derivative, gain on derivative	$ 3,300,000	$ 0
2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Senior notes interest rate	2.25%
3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Senior notes interest rate	3.15%
Senior Notes
Debt Instrument [Line Items]
Face amount			$ 3,000,000,000.0
Debt instrument, redemption price, percentage	100.00%
Debt instrument, make whole provision redemption price, percentage	101.00%
Senior Notes \| 2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Face amount			$ 1,500,000,000
Senior notes interest rate			2.25%
Percentage par value of senior notes			99.973%
Senior Notes \| 3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Face amount			$ 1,500,000,000
Senior notes interest rate			3.15%
Percentage par value of senior notes			99.174%
Senior Notes \| 2.90% Senior Notes Due Sept 15, 2020
Debt Instrument [Line Items]
Face amount				$ 1,500,000,000
Senior notes interest rate				2.90%
Percentage par value of senior notes				99.792%
Payment of early call premium and write off of remaining unamortized debt issuance costs	$ 12,700,000
Interest Expense \| Senior Notes \| 2.90% Senior Notes Due Sept 15, 2020
Debt Instrument [Line Items]
Pre-tax charge, payment of early call premium and write off of remaining unamortized debt issuance costs, net of derivative gain	9,400,000
Interest Expense \| Interest rate swap
Debt Instrument [Line Items]
Derivative, gain on derivative	$ 3,300,000