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Collaborative and Other Relationships - Summary of Activity Related to BAN2401 and Elenbecestat Collaboration (Details) - Eisai - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of BAN2401 and elenbecestat	$ 219.3	$ 348.7	$ 232.0
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
Selling, general and administrative | E2609 and BAN2401
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	4.9	16.2	5.4
Total sales and marketing expense incurred by the collaboration	$ 9.8	$ 32.4	$ 10.7

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