Additional reduction in royalty rate payable on commercial sales

No definition available.

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

- Definition

Investment in biotechnology companies that are determined to be unconsolidated variable interest entities.

+ References

No definition available.

+ Details

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

- Definition

Investment in Variable Interest Entities.

+ References

No definition available.

+ Details

Name:	biib_InvestmentInVariableInterestEntitiesTextualAbstract
Namespace Prefix:	biib_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Reduction in royalty rate payable on commercial sales

+ References

No definition available.

+ Details

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

- Definition

Amount of cash outflow to a noncontrolling interest. Includes, but not limited to, reduction of noncontrolling interest ownership. Excludes dividends paid to the noncontrolling interest.

+ 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=109223946&loc=d3e3291-108585

+ Details

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

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

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=biib_NeurimmuneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Oct. 31, 2017	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Payments to Neurimmune			$ 38.9	$ 0.0
Investment in Variable Interest Entities (Textual)
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities		$ 28.1	$ 28.1		$ 48.3
Neurimmune
Variable Interest Entity [Line Items]
Payments to Neurimmune	$ 150.0	$ 50.0
Reduction in royalty rate payable on commercial sales	15.00%
Additional reduction in royalty rate payable on commercial sales		5.00%