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Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Eisai |  |  |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |  |  |
Percentage of future development costs related to Eisai |  |  |  | 45.00% |  | 15.00%
Loss on research and development contracts terminated with Eisai |  | $ 45.0 |  |  |  |
Eisai | BAN2401 and Elenbecestat |  |  |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |  |  |
Expense incurred by the collaboration | $ 68.6 |  | $ 54.6 | $ 136.6 | $ 111.1 |
Expense reflected within statements of income | 34.3 |  | 27.3 | 68.3 | 55.5 |
Skyhawk Therapeutics |  |  |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |  |  |
Additional milestone payment to Skyhawk | 2,000.0 |  |  | 2,000.0 |  |
Upfront and milestone payments made to collaborative partner |  | 74.0 |  |  |  |
Research and development expense |  | 38.5 |  |  |  |
Prepaid research and development expenses |  | $ 35.5 |  |  |  |
Research and development | Eisai | Aducanumab |  |  |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |  |  |
Expense incurred by the collaboration | 3.2 |  | 76.5 | 165.8 | 140.0 |
Expense reflected within statements of income | 1.7 |  | 65.0 | 91.2 | 128.5 |
Selling, general and administrative | Eisai | Aducanumab |  |  |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |  |  |
Expense incurred by the collaboration | 0.2 |  | 14.0 | 21.2 | 21.2 |
Expense reflected within statements of income | $ 0.1 |  | $ 9.2 | $ 11.7 | $ 13.9 |

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