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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
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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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