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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2022
Variable Interest Entity [Line Items] |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  |  | $ 9,300.0 |  |  |
Income (loss) from equity method investments |  |  | 2.6 | $ 34.9 | $ 3.3 |
Deferred tax asset, net |  |  |  | 100.0 |  |
Valuation allowance |  |  | 2,003.3 | 1,961.3 |  | $ 85.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities |  |  | 27.8 | 24.6 |  |
ADUHELM |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments |  |  | $ 111.0 | 30.0 |  |
Neurimmune |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |
Term of collaboration agreement |  |  | 12 years |  |  |
Research and development costs, percentage |  |  | 100.00% |  |  |
Regulatory Milestones |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  |  | $ 500.0 |  |  |
Regulatory Milestones | Neurimmune |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  | $ 75.0 |  |  |  |
Eisai |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments |  | $ 33.8 | $ 0.0 | 45.0 | $ 33.8 |
Eisai | ADUHELM |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments | $ 45.0 |  |  | $ 59.0 |  |
Eisai | Regulatory Milestones | Neurimmune | ADUHELM |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately | $ 100.0 |  |  |  |  |

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