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Commitments and Contingencies (Details) (Fumapharm AG [Member], USD $) | Mar. 31, 2014 | Dec. 31, 2013
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Five hundred million [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Cumulative sales level | $ 500,000,000 |
One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Cumulative sales level | 1,000,000,000.0 | 1,000,000,000
Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Cumulative sales level | 2,000,000,000.0 |
Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Cumulative sales level | 3,000,000,000.0 |
Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Cumulative sales level | 20,000,000,000.0 |
Each additional sales level | 1,000,000,000.0 |
Less than five hundred million [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 500,000,000 |
Less than five hundred million [Member] | One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Less than five hundred million [Member] | Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Less than five hundred million [Member] | Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Less than five hundred million [Member] | Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Five hundred million to one billion [Member] | One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 25,000,000 |
Five hundred million to one billion [Member] | Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 50,000,000 |
Five hundred million to one billion [Member] | Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 50,000,000 |
Five hundred million to one billion [Member] | Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 50,000,000 |
One billion to one point five billion [Member] | One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 50,000,000 |
One billion to one point five billion [Member] | Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 100,000,000 |
One billion to one point five billion [Member] | Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 100,000,000 |
One billion to one point five billion [Member] | Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 100,000,000 |
One point five billion to two billion [Member] | One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
One point five billion to two billion [Member] | Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 150,000,000 |
One point five billion to two billion [Member] | Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 150,000,000 |
One point five billion to two billion [Member] | Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 150,000,000 |
Two billion to two point five billion [Member] | One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Two billion to two point five billion [Member] | Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 200,000,000 |
Two billion to two point five billion [Member] | Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 200,000,000 |
Two billion to two point five billion [Member] | Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 200,000,000 |
Two point five billion to three billion [Member] | One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Two point five billion to three billion [Member] | Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Two point five billion to three billion [Member] | Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 250,000,000 |
Two point five billion to three billion [Member] | Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 250,000,000 |
Greater than three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 3,000,000,000 |
Greater than three billion [Member] | One billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Greater than three billion [Member] | Two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Greater than three billion [Member] | Three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 0 |
Greater than three billion [Member] | Each additional one billion up to twenty billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Milestone payment due on the prior 12 month sales | 300,000,000 |
Minimum | Five hundred million to one billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 500,000,000 |
Minimum | One billion to one point five billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 1,000,000,000 |
Minimum | One point five billion to two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 1,500,000,000.0 |
Minimum | Two billion to two point five billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 2,000,000,000 |
Minimum | Two point five billion to three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 2,500,000,000.0 |
Maximum | Five hundred million to one billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 1,000,000,000 |
Maximum | One billion to one point five billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 1,500,000,000.0 |
Maximum | One point five billion to two billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 2,000,000,000 |
Maximum | Two billion to two point five billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | 2,500,000,000.0 |
Maximum | Two point five billion to three billion [Member] |  |
Business Acquisition, Contingent Consideration [Line Items] |  |
Sales level | $ 3,000,000,000 |

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- Definition Cumulative sales level + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Each additional sales level + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Milestone payment due on the prior 12 month sales + References No definition available. + Details
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Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Sales level + References No definition available. + Details
Name: | biib_SalesLevel
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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Period Type: | duration