Collaborative and Other Relationships Collaborative and Other Relationships (Details 3) (Eisai, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total expense incurred by collaboration	$ 57.5	$ 0	$ 0
Biogen Idec's share of expense reflected within our consolidation statements of income	$ 29.1	$ 0	$ 0