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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended |
--- | --- | --- | ---
May 31, 2018 | Oct. 31, 2017 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Variable Interest Entity [Line Items] |  |  |  |  |
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities |  |  |  |  | $ 12.8
Loss on equity method investment |  |  | $ (18.2) | $ (16.8) |
Neurimmune |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |
Collaboration agreement term |  |  | 12 years |  |
Research and development costs, percentage |  |  | 100.00% |  |
Payments to Noncontrolling Interests | $ 50.0 | $ 150.0 |  |  |
Reduction in royalty rate payable on commercial sales |  | 15.00% |  |  |
Additional reduction in royalty rate payable on commercial sales | 5.00% |  |  |  |
Twelve months |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |
Potential Future Milestone Payments Commitment To Third Party Approximately |  |  | $ 150.0 |  |
Regulatory Milestones [Member] | Neurimmune |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |
Potential Future Milestone Payments Commitment To Third Party Approximately |  |  |  |  | $ 75.0
Eisai |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |
Loss on equity method investment |  |  | $ 33.8 |  |
Percentage of future development costs related to Eisai |  |  | 45.00% |  |
Aducanumab | Regulatory Milestones [Member] | JAPAN |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |
Potential Future Milestone Payments Commitment To Third Party Approximately |  |  | $ 50.0 |  |
Neurimmune | Regulatory Milestones [Member] |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |
Potential Future Milestone Payments Commitment To Third Party Approximately |  |  | $ 100.0 |  |

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