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Commitments and Contingencies (Details) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Oct. 31, 2019 | Dec. 31, 2022 | Dec. 31, 2016 | Dec. 31, 2023 | Dec. 31, 2021 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2013
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately |  | $ 9,300.0 |  |  |  |  |  |
Accrued expenses |  | 20.4 |  |  |  |  |  |
Cancellable future commitments |  | 929.0 |  |  |  |  |  |
Liabilities associated with uncertain tax positions |  | 154.6 |  |  |  |  |  |
Income taxes paid |  | 137.8 |  |  |  |  |  |
2017 Tax Act |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Transition toll tax liabilities |  | 558.0 |  |  | $ 633.0 |  |  |
Development Milestones |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately |  | 2,000.0 |  |  |  |  |  |
Regulatory Milestones |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately |  | 500.0 |  |  |  |  |  |
Regulatory Milestones | Neurimmune |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately |  |  |  |  |  |  | $ 75.0 |
Commercial Milestones |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately |  | $ 6,800.0 |  |  |  |  |  |
Ionis Pharmaceuticals | Minimum | SPINRAZA |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of royalties as per collaboration |  | 11.00% | 11.00% |  |  |  |  |
Ionis Pharmaceuticals | Maximum | SPINRAZA |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of royalties as per collaboration |  | 15.00% | 15.00% |  |  |  |  |
Alkermes | VUMERITY |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of royalties as per collaboration | 15.00% | 15.00% |  |  |  |  |  |
Royalties payable period, post FDA approval | 5 years |  |  |  |  |  |  |
Aducanumab | Forecast |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately |  |  |  | $ 356.2 |  |  |  |
Eisai | ADUHELM | Regulatory Milestones | Neurimmune |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately |  |  |  |  |  | $ 100.0 |  |
TYSABRI |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Future contingent payment for annual worldwide net sales up to threshold, percentage |  |  |  |  |  |  |  | 18.00%
Future contingent payment threshold |  |  |  |  |  |  |  | $ 2,000.0
Future contingent payment for annual worldwide net sales that exceeding threshold, percentage |  |  |  |  |  |  |  | 25.00%

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