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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
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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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