Equity Method Investment, Ownership Percentage Before Additional Purchase Transaction

No definition available.

Name:	biib_EquityMethodInvestmentOwnershipPercentageBeforeAdditionalPurchaseTransaction
Namespace Prefix:	biib_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Investment in common stock, shares purchased

+ References

No definition available.

+ Details

Name:	biib_Investmentincommonstocksharespurchased
Namespace Prefix:	biib_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Purchase of common stock

+ References

No definition available.

+ Details

Name:	biib_Purchaseofcommonstock
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Strategic investment portfolio.

+ References

No definition available.

+ Details

Name:	biib_StrategicInvestmentPortfolio
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Term of collaboration agreement

+ References

No definition available.

+ Details

Name:	biib_Termofcollaborationagreement
Namespace Prefix:	biib_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Total payment to enter collaboration agreement

+ References

No definition available.

+ Details

Name:	biib_Totalpaymenttoentercollaborationagreement
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Upfront payment for collaboration agreement

+ References

No definition available.

+ Details

Name:	biib_Upfrontpaymentforcollaborationagreement
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8813-108599

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_EquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_StrategicInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_IonisPharmaceuticalsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=biib_SamsungBiosimilarAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments (Details Textual 2) shares in Millions, $ in Millions, ₩ in Billions	3 Months Ended				12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 KRW (₩)	Jun. 30, 2018	Sep. 30, 2013	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 KRW (₩)	Jun. 05, 2018 USD ($) shares	Dec. 31, 2012 USD ($)	Feb. 29, 2012 USD ($)	Feb. 29, 2012 KRW (₩)
Business Acquisition [Line Items]
Payments to Acquire Equity Method Investments					$ 676.6	$ 0.0	$ 0.0
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 676.3				$ 676.3	$ 85.8
Ionis Pharmaceuticals
Business Acquisition [Line Items]
Term of collaboration agreement			10 years	6 years
Total payment to enter collaboration agreement									$ 1,000.0
Upfront payment for collaboration agreement									$ 375.0	$ 30.0
Investment in common stock, shares purchased \| shares									11.5
Purchase of common stock									$ 625.0
Samsung Bioepis
Business Acquisition [Line Items]
Equity Method Investment, Ownership Percentage Before Additional Purchase Transaction	5.00%				5.00%			5.00%
Equity Method Investment, Ownership Percentage	49.90%				49.90%			49.90%			15.00%	15.00%
Payments to Acquire Equity Method Investments	$ 676.6	₩ 759.5
Equity Method Investments	$ 680.6				$ 680.6			₩ 759.5			$ 45.0	₩ 49.5