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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
(Gain) loss on equity method investments |  |  |  |  |  |  | $ 2.6 | $ 16.1
Deferred tax assets, net |  |  |  |  | $ 490.0 |  |  |
Deferred tax assets, unrecognized tax benefit |  | $ 85.0 |  | $ 390.0 |  |  | 85.0 |
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities |  | 23.5 |  | 24.6 |  |  | $ 23.5 |
Neurimmune |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Collaboration agreement term |  |  |  |  |  |  | 12 years |
Research and development costs, percentage |  |  |  |  |  |  | 100.00% |
Eisai |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Collaboration agreement term |  |  | 5 years |  |  |  | 10 years |
(Gain) loss on equity method investments |  | 28.9 |  |  |  | $ 40.1 | $ 210.6 | 40.1
Percentage of future development costs related to Eisai |  |  |  |  |  | 45.00% | 45.00% |
Eisai | ADUHELM |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
(Gain) loss on equity method investments |  | 160.0 |  | $ 59.0 | 45.0 | $ 45.0 |  |
Additional milestone payment | $ 100.0 | $ 0.0 |  |  |  | $ 100.0 | $ 0.0 | $ 100.0
Eisai | Regulatory Milestones | Neurimmune | ADUHELM |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party |  |  |  |  | $ 100.0 |  |  |

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