Collaborative and Other Relationships (Details 4) (Portola Pharmaceuticals, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Portola Pharmaceuticals
Summary of activity related to collaboration with Portola Pharmaceuticals, Inc.
Total expense incurred by collaboration	$ 1.6	$ 18.8	$ 1.1
Biogen Idec's share of expense reflected within our consolidation statements of income	$ 1.6	$ 14.2	$ 0.9