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Collaborative and Other Relationships - Summary of Activity Related to the UCB Collaboration (Details) - Research and development - UCB Pharma S.A. - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Total UCB development expense	$ 58.3	$ 31.9	$ 29.7
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	$ 29.2	$ 16.0	$ 14.9

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