Collaborative and Other Relationships (Details 1) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Collaborative arrangements and non-collaborative arrangement transactions
Revenues	$ 2,872.0	$ 2,955.8	$ 2,894.2	$ 2,726.8	$ 2,839.3	$ 2,777.9	$ 2,591.6	$ 2,555.0	$ 11,448.8	$ 10,763.8	$ 9,703.3
AbbVie
Collaborative arrangements and non-collaborative arrangement transactions
Revenues									6.1
Total expense incurred by collaboration									50.0
Co-promotion profits and losses									43.9
Biogen's share of co-promotion losses									$ 21.9