Name:	biib_DerivativeInstrumentsTextualAbstract
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Amount of net unrealized gain (loss) related to the change in fair value of foreign currency exchange rate derivatives designated as cash flow hedging instruments. Recorded in accumulated other comprehensive income to the extent that the cash flow hedge is determined to be effective.

+ References

No definition available.

+ Details

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

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- Definition

Fair value before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226000-175313

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959

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- Definition

Fair value before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

+ References

+ Details

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- Definition

High end period remaining until maturity for the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4D

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624177-113959

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- Definition

Low end period remaining until maturity of derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Amount of gain (loss) from the ineffectiveness of the hedge.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 25

-Section 50

-Paragraph 1

-Subparagraph (a)(1)

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- Definition

Amount of gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of foreign currency derivatives not designated as hedging instruments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959

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- Definition

Maximum amount of time remaining before foreign currency exchange rate derivatives mature or expire, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative Instruments (Textual) [Abstract]
Lower Range of Durations of foreign currency forward contracts			1 month		1 month
Higher Range of Durations of foreign currency forward contracts			15 months		12 months
Gain/Loss on fair value of Foreign currency forward contracts	$ 16.2		$ 16.2		$ 11.8
Expected Settlement time for contracts, in months			15 months
Net gains (losses) of other income (expense) related to foreign currency forward contracts	0.2	5.7	1.3	5.6
Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	580.6		580.6		524.0
Not Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	176.7		176.7		243.2
Foreign Exchange Contract [Member] \| Other Current Assets [Member] \| Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Asset, Fair Value, Net	0.4		0.4		0.6
Foreign Exchange Contract [Member] \| Other Current Assets [Member] \| Not Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Asset, Fair Value, Net	1.4		1.4		1.2
Foreign Exchange Contract [Member] \| Accrued Liabilities [Member] \| Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Liability, Fair Value, Net	15.4		15.4		11.5
Foreign Exchange Contract [Member] \| Accrued Liabilities [Member] \| Not Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Liability, Fair Value, Net	1.7		1.7		2.9
Euro [Member] \| Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	570.0		570.0		492.2
Canadian Dollar [Member] \| Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	10.6		10.6		31.8
Sales [Member] \| Foreign Exchange Contract [Member]
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in product revenue for the settlement of certain effective cash flow hedge instruments	(7.3)	12.0	(6.1)	31.0
Other Income Expense [Member] \| Foreign Exchange Contract [Member]
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness	$ (0.1)	$ 0.8	$ 0.2	$ 4.0