Collaborative and Other Relationships Collaborative and Other Relationships - Eisai (Details 1) - Eisai - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs related to Eisai	15.00%	45.00%
BAN2401 and Elenbecestat R&D expense incurred by collaboration		$ 348.7	$ 232.0	$ 146.2
Biogen's share of expense reflected within our consolidation statements of income		174.3	116.0	74.3
Research and development | Aducanumab
Collaborative arrangements and non-collaborative arrangement transactions
Aducanumab expense incurred by the collaboration		179.4	264.8	268.7
Biogen's share of expense reflected within our consolidation statements of income		98.7	234.6	268.7
Selling, General and Administrative Expenses | Aducanumab
Collaborative arrangements and non-collaborative arrangement transactions
Aducanumab expense incurred by the collaboration		27.4	50.6	23.6
Biogen's share of expense reflected within our consolidation statements of income		$ 15.1	$ 27.3	$ 23.6