Collaborative and Other Relationships - Acorda (Details) - Acorda - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Expected additional milestone payments when certain sales threshold is met	$ 15.0
Foreign sales required to trigger milestone	100.0
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	$ 44.5	$ 42.0	$ 36.5

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