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v3.20.4

Fair Value Measurements - Gains and Losses on Assets Measured at Fair Value (Details) $ in Millions | 7 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Jun. 07, 2019 USD ($)
Finite-lived Intangible Assets [Roll Forward] |  |  |
Beginning Book Value | $ 8,220.8 | $ 8,408.8 |
Ending Book Value | 3,084.3 | $ 3,527.4 |
BIIB111 | Fair Value, Nonrecurring |  |  |
Finite-lived Intangible Assets [Roll Forward] |  |  |
Beginning Book Value |  |  | $ 480.0
Impairment | (115.0) |  |
Ending Book Value | $ 365.0 |  |

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