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Reserves for Discounts and Allowances (Details 1) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Summary of total product revenue reserves included in consolidated balance sheets
Reduction of accounts receivable	$ 134.5	$ 151.4
Component of accrued expenses and other	330.5	264.9
Total reserves	$ 465.0	$ 416.3

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- Definition

Period balance related to product revenue reserves.

+ References

No definition available.

+ Details

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

X

- Definition

Product revenue reserves classified as a reduction of accounts receivable.

+ References

No definition available.

+ Details

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

X

- Definition

Product revenue reserves classified as a reduction of current liabilities.

+ References

No definition available.

+ Details

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

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- Details

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Period Type:	duration