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Inventory - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2022 | Dec. 31, 2021 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Inventory [Line Items] |  |  |  |  |  |  |
Write-downs on excess, obsolete, unmarketable or other inventory |  |  |  |  | $ 336.2 | $ 167.6 | $ 26.6
Inventory |  | $ 1,351.5 |  |  | 1,365.5 | 1,351.5 |
Income (loss) from equity method investments |  |  |  |  | 2.6 | 34.9 | 3.3
Eisai |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | $ 33.8 | 0.0 | 45.0 | $ 33.8
ADUHELM |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
Write-downs on excess, obsolete, unmarketable or other inventory | $ 275.0 | 120.0 |  |  |  |  |
Inventory |  | $ 223.0 |  |  |  | 223.0 |
Income (loss) from equity method investments |  |  |  |  | 111.0 | 30.0 |
ADUHELM | Eisai |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  | $ 45.0 |  |  | $ 59.0 |
ADUHELM | Centers for Medicare and Medicaid Service | Eisai |  |  |  |  |  |  |
Inventory [Line Items] |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  |  | $ 136.0 |  |

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