Financial Instruments (Details Textual 1) (USD $)	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financial Instruments (Textual)
Acquisition of TYSABRI rights	$ 3,262,719,000	$ 0	$ 0
Original maturities of commercial paper and short-term debt securities	less than 90 days
Average maturity of marketable securities, months	13 months	14 months
TYSABRI product
Financial Instruments (Textual)
Acquisition of TYSABRI rights	3,250,000,000
Strategic Investments
Financial Instruments (Textual)
Impairment charges	$ 2,800,000	$ 5,500,000	$ 9,900,000

Average remaining time to maturity of marketable debt securities, available-for-sale securities.

No definition available.

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

- Details

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

- Definition

Impairment charges related to investments in privately held companies.

+ References

No definition available.

+ Details

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

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

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

+ Details

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

- Definition

The cash outflow to acquire asset without physical form usually arising from contractual or other legal rights, excluding goodwill.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

+ Details

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