Collaborative and Other Relationships - Eisai (Details) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2021 USD ($) product	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Income (loss) from equity method investments				$ 34.9	$ 3.3	$ (77.4)
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Number of product candidates | product				2
Loss on contract termination	$ 45.0
Percentage of future development costs				45.00%
Income (loss) from equity method investments		$ 33.8	$ 33.8	$ 45.0	33.8	0.0
Additional milestone payment			$ 75.0	$ 100.0	$ 75.0	$ 0.0
Eisai | United States
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses				55.00%
Eisai | European Union
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses				68.50%
Eisai | JAPAN
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses				20.00%