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Collaborative and Other Relationships - Eisai (Details) $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended			24 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2020 USD ($) product	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of development costs incurred			100.00%
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income					$ 3.3	$ (77.4)	$ 0.0
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 related to Eisai				15.00%	45.00%			45.00%
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income					$ 33.8	0.0	0.0
Total aducanumab collaboration third party milestone expense	$ 75.0				$ 75.0	$ 0.0	$ 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%

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