Collaborative and Other Relationships Collaborative and Other Relationships - Eisai (Details 1) - Eisai - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative arrangements and non-collaborative arrangement transactions
BAN2401 and Elenbecestat R&D expense incurred by collaboration	$ 232.0	$ 146.2	$ 95.1
Biogen's share of expense reflected within our consolidation statements of income	116.0	74.3	50.5
Research and development | Aducanumab [Member]
Collaborative arrangements and non-collaborative arrangement transactions
Aducanumab expense incurred by the collaboration	264.8	268.7	209.5
Biogen's share of expense reflected within our consolidation statements of income	234.6	268.7	209.5
Selling, General and Administrative Expenses [Member] | Aducanumab [Member]
Collaborative arrangements and non-collaborative arrangement transactions
Aducanumab expense incurred by the collaboration	50.6	23.6	18.3
Biogen's share of expense reflected within our consolidation statements of income	$ 27.3	$ 23.6	$ 18.3