Reserves for Discounts and Allowances (Details 1) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of total product revenue reserves included in consolidated balance sheets
Reduction of accounts receivable	$ 48.4	$ 46.1
Component of accrued expenses and other	204.0	191.0
Total reserves	$ 252.4	$ 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