Accrued expenses on funding commitment related to clinical research.

No definition available.

Name:	biib_AccruedExpensesOnFundingCommitmentRelatedToClinicalResearch
Namespace Prefix:	biib_
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

- Details

Name:	biib_CommitmentsAndContingenciesTextualAbstract
Namespace Prefix:	biib_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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 payment due on the prior 12 month sales

+ References

No definition available.

+ Details

Name:	biib_MilestonePaymentDueOnPrior12MonthSales
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
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
Namespace Prefix:	us-gaap_
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

Name:	us-gaap_GoodwillAcquiredDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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

Name:	us-gaap_LiabilityForUncertainTaxPositionsCurrent
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=31042434&loc=d3e3213-108585

+ Details

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

- Details

Name:	biib_CumulativeSalesLevelAxis=biib_EachAdditionalOneBillionUpToTwentyBillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_CumulativeSalesLevelAxis=biib_FourbillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_CumulativeSalesLevelAxis=biib_OneBillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_CumulativeSalesLevelAxis=biib_ThreeBillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_CumulativeSalesLevelAxis=biib_TwoBillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_FacilityLocationAxis=biib_CambridgeLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_ProductOrServiceAxis=biib_TecfideraMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies (Details Textual) (USD $)	12 Months Ended			3 Months Ended	12 Months Ended
Commitments and Contingencies (Details Textual) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014	Dec. 31, 2010	Dec. 31, 2006	Apr. 02, 2013	Jan. 31, 2007	Jun. 30, 2013
Business Acquisition
Increase to goodwill	$ 527,300,000us-gaap_GoodwillAcquiredDuringPeriod	$ 35,700,000us-gaap_GoodwillAcquiredDuringPeriod
Commitments And Contingencies (Textual)
Lease rent expense which terminates at various dates through 2028	62,400,000us-gaap_LeaseAndRentalExpense	56,100,000us-gaap_LeaseAndRentalExpense	49,000,000us-gaap_LeaseAndRentalExpense
Liabilities associated with uncertain tax positions	80,200,000
Accrued expenses	41,600,000
Cancellable future commitments	472,300,000us-gaap_OtherCommitment
Potential future milestone payments commitment, approximately	2,800,000,000
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							2,000,000,000
Future contingent payment for annual worldwide net sales that exceed $2.0 billion							25.00%
Biogen Idec Hemophilia
Business Acquisition
Maximum contingent consideration in the form of development and approval milestones								80,000,000
Milestone payments made during period				20,000,000	40,000,000
Fumapharm AG
Business Acquisition
Amount paid in cash						220,000,000
Cambridge Leases
Business Acquisition
Charges recognized to vacate building		27,200,000
TECFIDERA \| Fumapharm AG
Business Acquisition
Maximum contingent consideration in the form of development and approval milestones									15,000,000
Increase to goodwill	600,000,000
One billion \| Fumapharm AG
Business Acquisition
Cumulative sales level	1,000,000,000
One billion \| TECFIDERA \| Fumapharm AG
Business Acquisition
Amount paid in cash	25,000,000
Two billion \| Fumapharm AG
Business Acquisition
Cumulative sales level	2,000,000,000
Two billion \| TECFIDERA \| Fumapharm AG
Business Acquisition
Increase to goodwill	150,000,000
Three billion \| Fumapharm AG
Business Acquisition
Cumulative sales level	3,000,000,000
Three billion \| TECFIDERA \| Fumapharm AG
Business Acquisition
Increase to goodwill	200,000,000
Four billion \| Fumapharm AG
Business Acquisition
Cumulative sales level	4,000,000,000
Four billion \| TECFIDERA \| Fumapharm AG
Business Acquisition
Increase to goodwill	250,000,000
Each additional one billion up to twenty billion \| Fumapharm AG
Business Acquisition
Cumulative sales level	20,000,000,000
Each additional one billion up to twenty billion \| TECFIDERA \| Fumapharm AG
Business Acquisition
Future contingent payments due	$ 300,000,000