Milestone Payments Made During Period.

No definition available.

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

- Definition

Percentage of venture capital investments to assets.

+ References

No definition available.

+ Details

Name:	biib_PercentageOfVentureCapitalInvestmentsToAssets
Namespace Prefix:	biib_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Venture capital investments as a percentage of our total assets.

+ References

No definition available.

+ Details

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Namespace Prefix:	biib_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Details

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- Definition

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 35

-Paragraph 1

-Subparagraph b

-URI http://asc.fasb.org/extlink&oid=6911338&loc=d3e6819-128478

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of liability recognized arising from contingent consideration in a business combination, expected to be settled beyond one year or the normal operating cycle, if longer.

+ References

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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)(1))

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

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ 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 (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Description of the changes in valuation technique(s) and related inputs used to measure fair value.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementsChangesInValuationTechniques
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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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=36462937&loc=d3e19207-110258

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=biib_BiogenIdecInternationalNeuroscienceMember
Namespace Prefix:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=us-gaap_OtherNoncurrentLiabilitiesMember
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- Details

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Fair Value Measurements (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended		3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2012	Sep. 30, 2011	Dec. 31, 2010	Mar. 08, 2012
Business Acquisition, Contingent Consideration [Line Items]
Venture capital investments as percentage of our assets	approximately 0.1% of total assets	approximately 0.2% of total assets
Percentage of venture capital investments to assets	0.10%	0.20%
Contingent consideration obligations	$ 200.0	$ 251.9
Additions	0	0
Fair value measurements, changes in valuation techniques	0	0
Stromedix, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Additions			122.2
Contingent consideration obligations	130.5	140.7
Maximum contingent consideration in the form of development and approval milestones	419.0					487.5
Discount rate used for net cash outflow projections for fair value measurement	3.00%
Total purchase price allocated to IPR&D						219.2
Biogen Dompe SRL and Biogen Dompe Switzerland GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions				38.8
Contingent consideration obligations	15.5	31.6
Maximum contingent consideration in the form of development and approval milestones	22.0
Discount rate used for net cash outflow projections for fair value measurement	2.00%
Milestone payments made during period	16.5
Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions					81.2
Contingent consideration obligations	69.5	108.6
Maximum contingent consideration in the form of development and approval milestones	362.5
Discount rate used for net cash outflow projections for fair value measurement	3.60%
Milestone payments made during period	10.0
6.875% Senior Notes due 2018
Business Acquisition, Contingent Consideration [Line Items]
Interest rate on senior notes	6.875%
Other long-term liabilities
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations	$ 200.0	$ 251.9