Base rate after first commercial sale.

No definition available.

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

- Definition

Future research and development commitment

+ References

No definition available.

+ Details

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

- Definition

Potential future milestone payments commitment to third party approximately.

+ References

No definition available.

+ Details

Name:	biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Rate during the reimbursement period.

+ References

No definition available.

+ Details

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

- Definition

Reimbursement cost achieving period.

+ References

No definition available.

+ Details

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

- Definition

Reimbursement, under the amended agreement, method.

+ References

No definition available.

+ Details

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

- Definition

Reimbursement recognition period

+ References

No definition available.

+ Details

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

- Definition

Time period for paying remaining balance due.

+ References

No definition available.

+ Details

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

- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65885119&loc=d3e1392-128463

+ Details

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

- Definition

Amount of noncurrent assets classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The costs incurred in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, excluding in-process research and development acquired in a business combination consummated during the period. Excludes software research and development, which has a separate concept.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Research and Development
-URI http://asc.fasb.org/extlink&oid=6523717

+ Details

Name:	us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis=biib_SobiRateToBiogenOnNetSalesInSobiTerritoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis=biib_BiogenRateToSobiOnNetSalesInBiogenNorthAmericaTerritoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=biib_SobiTerritoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis=biib_BiogenRateToSobiOnNetSalesInBiogenDirectTerritoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis=biib_BiogenRateToSobiOnNetRevenueFromBiogenDistributorTerritoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=biib_BiogenIdecHemophiliaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_ProductOrServiceAxis=biib_ELOCTATEMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=biib_ALPROLIXMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaborative and Other Relationships Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Jan. 31, 2007
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 473.1	$ 490.7	$ 910.4	$ 951.3
Investments and other assets	1,153.0		$ 1,153.0		$ 1,107.6
Sobi rate to Biogen on net sales in the Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method			Royalty
Base rate after first commercial sale			12.00%
Rate during the reimbursement period			Base Rate plus 5%
Biogen rate to Sobi on net sales in the Biogen North America Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method			Royalty
Rate during the reimbursement period			Base Rate less 5%
Biogen rate to Sobi on net sales in the Biogen North America Territory \| Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Base rate after first commercial sale			12.00%
Biogen rate to Sobi on net sales in the Biogen Direct Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method			Royalty
Rate during the reimbursement period			Base Rate less 5%
Biogen rate to Sobi on net sales in the Biogen Direct Territory \| Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Base rate after first commercial sale			17.00%
Biogen rate to Sobi on net revenue from the Biogen Distributor Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method			Net Revenue Share
Rate during the reimbursement period			Base Rate less 15%
Biogen rate to Sobi on net revenue from the Biogen Distributor Territory \| Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Base rate after first commercial sale			50.00%
Biogen Hemophilia
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of stock acquired						100.00%
University of Pennsylvania
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	20.0
Investments and other assets	15.0		$ 15.0
Future research and development commitment	47.5		47.5
Potential future milestone payments	$ 2,000.0		$ 2,000.0
Swedish Orphan Biovitrum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement recognition period			10 years
Reimbursement cost achieving period			6 years
Time period for paying remaining balance due			90 days
Swedish Orphan Biovitrum \| ELOCTATE
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development			$ 157.0
Swedish Orphan Biovitrum \| ALPROLIX
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development			$ 130.0