v2.4.0.6

Reserves for Discounts and Allowances (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended
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Sep. 30, 2012
Analysis of the amount of, and change in, reserves |
Balance, as of December 31, 2011 | $ 155.6
Current provisions relating to sales in current year | 455.1
Adjustments relating to prior years | (5.8)
Payments/returns relating to sales in current year | (291.5)
Payments/returns relating to sales in prior years | (104.2)
Balance, as of June 30, 2012 | 209.2
Discounts [Member] |
Analysis of the amount of, and change in, reserves |
Balance, as of December 31, 2011 | 12.6
Current provisions relating to sales in current year | 84.4
Adjustments relating to prior years | (0.2)
Payments/returns relating to sales in current year | (70.8)
Payments/returns relating to sales in prior years | (11.0)
Balance, as of June 30, 2012 | 15.0
Contractual Adjustments [Member] |
Analysis of the amount of, and change in, reserves |
Balance, as of December 31, 2011 | 119.3
Current provisions relating to sales in current year | 353.4
Adjustments relating to prior years | (5.3)
Payments/returns relating to sales in current year | (217.4)
Payments/returns relating to sales in prior years | (83.2)
Balance, as of June 30, 2012 | 166.8
Returns [Member] |
Analysis of the amount of, and change in, reserves |
Balance, as of December 31, 2011 | 23.7
Current provisions relating to sales in current year | 17.3
Adjustments relating to prior years | (0.3)
Payments/returns relating to sales in current year | (3.3)
Payments/returns relating to sales in prior years | (10.0)
Balance, as of June 30, 2012 | $ 27.4

X
- Definition Adjustments to product revenue reserves relating to sales in prior years. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Current provisions to product revenue reserves relating to sales in current year. + References No definition available. + Details
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Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
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- Definition Payments/ returns applied against product revenue reserves relating to current year. + References No definition available. + Details
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Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Payments/ returns applied against product revenue reserves relating to prior year. + References No definition available. + Details
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Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Period balance related to product revenue reserves. + References No definition available. + Details
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Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant