Reserves for Discounts and Allowances (Details 1) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of total product revenue reserves included in consolidated balance sheets
Reduction of accounts receivable	$ 143.8	$ 46.1
Component of accrued expenses and other	247.4	191.0
Total reserves	$ 391.2	$ 237.1

Product revenue reserves classified as a reduction of accounts receivable.

No definition available.

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