Collaborative and Other Relationships (Details 1) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative arrangements and non-collaborative arrangement transactions
Product revenues, net	$ 3,307.0	$ 3,077.8	$ 3,078.4	$ 2,810.7	$ 2,872.0	$ 2,955.8	$ 2,894.2	$ 2,726.8	$ 12,273.9	$ 11,448.8	$ 10,763.8
Costs and expenses									6,929.7	6,298.4	$ 5,872.8
AbbVie
Collaborative arrangements and non-collaborative arrangement transactions
Product revenues, net									53.1	6.1
Costs and expenses									92.6	50.0
Co-promotion profits and losses									39.5	43.9
Biogen's share of co-promotion losses									$ 16.9	$ 21.9