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Inventory - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021
Inventory [Line Items] |  |  |  |  |  |  |
Excess and obsolescence charges related to inventory |  |  |  |  |  | $ 305.6 | $ 8.3
(Gain) loss on equity method investments |  |  |  |  |  | 2.6 | 16.1
Eisai |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
(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% |
ADUHELM |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
Excess and obsolescence charges related to inventory |  | $ 275.0 | $ 120.0 |  |  |  |
Inventory |  |  | 223.0 |  |  |  |
ADUHELM | Eisai |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
(Gain) loss on equity method investments | $ 160.0 |  | $ 59.0 | $ 45.0 | $ 45.0 |  |
ADUHELM | Eisai | Centers for Medicare and Medicaid Service |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
(Gain) loss on equity method investments |  |  |  |  |  | $ 136.0 |

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