v2.4.0.6

Fair Value Measurements (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair value of venture capital investments
Fair value, beginning of period	$ 25.4	$ 20.6	$ 23.5	$ 20.8
Unrealized gains included in earnings	1.4	1.8	4.9	2.5
Unrealized losses included in earnings	(1.6)	(0.2)	(3.6)	(1.5)
Purchases	0	0.9	0.4	1.3
Fair value, end of period	$ 25.2	$ 23.1	$ 25.2	$ 23.1

X

- Definition

Fair value measurement with unobservable inputs reconciliation recurring basis asset gain included in earnings.

+ References

No definition available.

+ Details

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Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Fair value measurement with unobservable inputs reconciliation recurring basis asset loss included in earnings.

+ References

No definition available.

+ Details

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Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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X

- Details

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

X

- Definition

Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

+ Details

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

X

- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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