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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2021 | May 31, 2018 | Oct. 31, 2017 | Sep. 30, 2021 | Jun. 30, 2021 | Sep. 30, 2020 | Jun. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |
Payments to Noncontrolling Interests |  |  |  |  |  |  |  | $ 94.8 | $ 70.9 |
(Gain) loss on equity method investment |  |  |  |  |  |  |  | 17.2 | (14.7) |
Deferred tax assets, net |  |  |  | $ 500.0 |  |  |  | 500.0 |  |
Deferred tax assets, gross | $ 875.0 |  |  |  | $ 875.0 |  |  |  |  |
Deferred tax assets, unrecognized tax benefit | 375.0 |  |  |  | $ 375.0 |  |  |  |  |
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities |  |  |  | 17.2 |  |  |  | $ 17.2 |  | $ 12.8
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% |  |  |  |  |  |  |  |
Regulatory Milestones | Neurimmune |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party |  |  |  |  |  |  | $ 75.0 |  |  |
Eisai |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |
(Gain) loss on equity method investment |  |  |  | 50.6 |  | $ 33.8 | $ 33.8 | $ 90.7 |  |
Percentage of future development costs related to Eisai |  |  |  |  | 45.00% | 45.00% | 45.00% |  |  |
Additional milestone payment |  |  |  |  |  | $ 75.0 |  |  |  |
Eisai | ADUHELM |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |
(Gain) loss on equity method investment |  |  |  | 45.0 | $ 45.0 |  |  | 45.0 |  |
Additional milestone payment | 100.0 |  |  | 0.0 |  | $ 0.0 |  | 100.0 | $ 75.0 |
Eisai | Regulatory Milestones | Neurimmune | ADUHELM |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party | $ 100.0 |  |  |  | $ 100.0 |  |  |  |  |
Eisai | Regulatory Milestones | JAPAN | ADUHELM |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party |  |  |  | 50.0 |  |  |  | 50.0 |  |
Eisai | Regulatory Milestones | Europe | ADUHELM |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party |  |  |  | $ 50.0 |  |  |  | $ 50.0 |  |

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