Accrued expenses on funding commitment related to clinical research.

No definition available.

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

- Definition

Charges recognized to vacate building

+ References

No definition available.

+ Details

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

- Definition

Commitments And Contingencies.

+ References

No definition available.

+ Details

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- Definition

Cumulative sales level

+ References

No definition available.

+ Details

Name:	biib_CumulativeSalesLevel
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Future contingent payment for annual worldwide net sales that exceed $2.0 billion

+ References

No definition available.

+ Details

Name:	biib_FutureContingentPaymentForAnnualWorldwideNetSalesThatExceed20Billion
Namespace Prefix:	biib_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Future contingent payment for annual worldwide net sales up to $2.0 billion

+ References

No definition available.

+ Details

Name:	biib_FutureContingentPaymentForAnnualWorldwideNetSalesUpTo20Billion
Namespace Prefix:	biib_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Future contingent payment threshold

+ References

No definition available.

+ Details

Name:	biib_FutureContingentPaymentThreshold
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Milestone Payments Made During Period.

+ References

No definition available.

+ Details

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

- Definition

Potential future milestone payments commitment to third party approximately.

+ References

No definition available.

+ Details

Name:	biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

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

- Definition

The current portion of the amount recognized for uncertain tax positions as of the balance sheet date.

+ 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 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

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

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ 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 (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3213-108585

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_BusinessAcquisitionAxis=biib_BiogenIdecHemophiliaMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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

- Details

Name:	biib_FacilityLocationAxis=biib_CambridgeLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	biib_FacilityLocationAxis=biib_SolothurnMember
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Data Type:	na
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- Details

Name:	us-gaap_ProductOrServiceAxis=biib_TecfideraMember
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Data Type:	na
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- Details

Name:	biib_CumulativeSalesLevelAxis=biib_SixbillionMember
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- Details

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Name:	biib_CumulativeSalesLevelAxis=biib_OneBillionMember
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- Details

Name:	biib_CumulativeSalesLevelAxis=biib_EachAdditionalOneBillionUpToTwentyBillionMember
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Commitments and Contingencies (Details Textual) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2010	Dec. 31, 2006	Jun. 30, 2013	Apr. 02, 2013	Jan. 31, 2007
Business Acquisition [Line Items]
Cancellable future commitments	$ 559.0		$ 559.0
Maximum contingent consideration in the form of development and approval milestones	1,300.0		1,300.0
Increase to goodwill			908.1	$ 527.3
Commitments And Contingencies (Textual)
Lease rent expense which terminates at various dates through 2028			68.6	62.4	$ 56.1
Liabilities associated with uncertain tax positions	45.4		45.4
Accrued expenses	25.0		25.0
Potential future milestone payments commitment, approximately	2,800.0		2,800.0
TYSABRI product
Commitments And Contingencies (Textual)
Future contingent payment for annual worldwide net sales up to $2.0 billion									18.00%
Future contingent payment threshold									$ 2,000.0
Future contingent payment for annual worldwide net sales that exceed $2.0 billion									25.00%
Biogen Hemophilia
Business Acquisition [Line Items]
Maximum contingent consideration in the form of development and approval milestones										$ 80.0
Milestone payments made during period		$ 20.0				$ 40.0
Fumapharm AG
Business Acquisition [Line Items]
Amount paid in cash							$ 220.0
Cambridge Leases
Business Acquisition [Line Items]
Charges recognized to vacate building					$ 27.2
Solothurn
Business Acquisition [Line Items]
Cancellable future commitments	126.4		126.4
TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Maximum contingent consideration in the form of development and approval milestones								$ 15.0
Increase to goodwill			900.0	$ 600.0
Six billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Amount paid in cash			850.0
Cumulative sales level	6,000.0		6,000.0
Four billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Cumulative sales level	4,000.0		4,000.0
Five billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Cumulative sales level	5,000.0		5,000.0
Seven billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Increase to goodwill	300.0
Cumulative sales level	7,000.0		7,000.0
One billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Cumulative sales level	1,000.0		1,000.0
Each additional one billion up to twenty billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Increase to goodwill	300.0
Cumulative sales level	20,000.0		20,000.0
Three billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Cumulative sales level	$ 3,000.0		$ 3,000.0