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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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