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Collaborative and Other Relationships Collaborative and Other Relationships - Eisai (Details 1) - Eisai - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |
Percentage of future development costs related to Eisai | 15.00% | 45.00% |  |
BAN2401 and Elenbecestat R&D expense incurred by collaboration |  | $ 348.7 | $ 232.0 | $ 146.2
Biogen's share of expense reflected within our consolidation statements of income |  | 174.3 | 116.0 | 74.3
Research and development | Aducanumab |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |
Aducanumab expense incurred by the collaboration |  | 179.4 | 264.8 | 268.7
Biogen's share of expense reflected within our consolidation statements of income |  | 98.7 | 234.6 | 268.7
Selling, General and Administrative Expenses | Aducanumab |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |
Aducanumab expense incurred by the collaboration |  | 27.4 | 50.6 | 23.6
Biogen's share of expense reflected within our consolidation statements of income |  | $ 15.1 | $ 27.3 | $ 23.6

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