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Collaborative and Other Relationships - Summary of Activity Related to Aducanumab Collaboration (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
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
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income		109.6	174.3	116.0
Total aducanumab collaboration third party milestone expense	$ 75.0	75.0	0.0	0.0
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income		33.8	0.0	0.0
Aducanumab | Research and development | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total UCB development expense		152.0	179.4	264.8
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income		83.6	98.7	234.6
Aducanumab | Selling, general and administrative | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total UCB development expense		353.0	27.4	50.6
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income		$ 193.7	$ 15.1	$ 27.3

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