XML 65 R62.htm IDEA: XBRL DOCUMENT
v3.3.0.814

Financial Instruments (Details Textual) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract] |  |
Original maturities of commercial paper and short-term debt securities | less than 90 days |
Overnight reverse repurchase agreements, percent of total assets | 0.90% | 2.10%
Average maturity of marketable securities, months | 13 months | 15 months

X
- Definition Average remaining time to maturity of marketable debt securities, available-for-sale securities. + References No definition available. + Details
Name: | biib_AverageMaturityOfMarketableSecurities
Namespace Prefix: | biib_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Overnight reverse repurchase agreements, percent of total assets + References No definition available. + Details
Name: | biib_Overnightreverserepurchaseagreementspercentoftotalassets
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- 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 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

X
- References No definition available. + Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration