Inventory Inventory (Details Textual) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Inventory, Various Programs, Gross	$ 35.8	$ 6.3

Gross inventory of various programs which have been capitalized in advance of regulatory approval.

No definition available.

Name:	biib_InventoryVariousProgramsGross
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant