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Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended
--- | --- | ---
Feb. 29, 2020 | Oct. 31, 2019 | Mar. 31, 2020 | Mar. 31, 2019
BAN2401 |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Research and development expense |  |  |  | $ 68.0
Elenbecestat Collaboration |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Research and development expense |  |  |  | 34.0
Eisai |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Expense Incurred By Collaboration |  |  | $ 43.5 |
Expense reflected within statements of income |  |  | $ 21.8 |
Percentage of future development costs related to Eisai |  |  | 45.00% |
Loss on research and development contracts terminated with Eisai |  |  |  | 45.0
UCB Pharma S.A. [Member] |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Expense Incurred By Collaboration |  |  | $ 19.0 |
Sangamo Therapeutics, Inc. Agreement [Member] |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Global Licensing Collaboration Agreement, Amount, Shares Purchased | $ 225.0 |  |  |
Global Licensing Collaboration Agreement, Shares, Purchased | 24 |  |  |
Global Licensing Collaboration Agreement, Shares Purchased Per Share | $ 9.21 |  |  |
Upfront and milestone payments made to collaborative partner | $ 125.0 |  |  |
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments | 2,400.0 |  |  |
Global Licensing Collaboration Agreement, Payment, Selection of Targets | 80.0 |  |  |
Global Licensing Collaboration Agreement, Payment, Achievement of First Commercial Sale Milestones | 1,900.0 |  |  |
Global Licensing Collaboration Agreement, Payment, Achievement of Specific Sales Based Milestones | 380.0 |  |  |
Research and development expense | $ 85.0 |  |  |
Skyhawk Therapeutics |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Additional Milestone Payment |  |  | 2,400.0 |
Upfront and milestone payments made to collaborative partner |  | $ 15.0 |  | 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 |  |  | 19.1 | 162.5
Expense reflected within statements of income |  |  | 10.5 | 89.4
Research and development | UCB Pharma S.A. [Member] |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Expense Incurred By Collaboration |  |  | 9.5 |
Selling, general and administrative | Eisai | BAN2401 and Elenbecestat |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Expense incurred by the collaboration |  |  | 4.9 |
Expense reflected within statements of income |  |  | 2.4 |
Selling, general and administrative | Eisai | Aducanumab |  |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |  |
Expense incurred by the collaboration |  |  | 22.7 | 20.9
Expense reflected within statements of income |  |  | $ 12.3 | $ 11.6

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