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Inventory - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2022 | Dec. 31, 2021 | Jun. 30, 2021 | Mar. 31, 2021
Inventory [Line Items] |  |  |  |
Excess and obsolescence charges related to inventory | $ 281.5 |  |  | $ 11.9
(Gain) loss on equity method investments | (3.3) |  |  | $ (18.2)
Eisai |  |  |  |
Inventory [Line Items] |  |  |  |
(Gain) loss on equity method investments | $ 181.7 |  |  |
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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