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Collaborative and Other Relationships - Eisai (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended | 24 Months Ended
--- | --- | --- | --- | ---
Jun. 30, 2020 USD ($) | Mar. 31, 2019 USD ($) | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2020 USD ($) product | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2020
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |
Percentage of development costs incurred |  |  | 100.00% |  |  |  |  |
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 |  |  |  |  |  |  |  |
Number of product candidates | product |  |  |  |  | 2 |  |  |
Loss on contract termination |  | $ 45.0 |  |  |  |  |  |
Percentage of future development costs related to Eisai |  |  |  | 15.00% | 45.00% |  |  | 45.00%
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income |  |  |  |  | $ 33.8 | 0.0 | 0.0 |
Total aducanumab collaboration third party milestone expense | $ 75.0 |  |  |  | $ 75.0 | $ 0.0 | $ 0.0 |
Eisai | United States |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |
Biogen share of co-promotion profits or losses |  |  |  |  | 55.00% |  |  |
Eisai | European Union |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |
Biogen share of co-promotion profits or losses |  |  |  |  | 68.50% |  |  |
Eisai | JAPAN |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |
Biogen share of co-promotion profits or losses |  |  |  |  | 20.00% |  |  |

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