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Fair Value Measurements - Nonrecurring Assets (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Dec. 31, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Fair Value, Assets Measured on Nonrecurring Basis, Reconciliation, Calculation [Roll Forward] |  |  |  |
Impairment of intangible assets |  |  | $ (585.9) | $ 0.0
BIIB111 | Nightstar |  |  |  |
Fair Value, Assets Measured on Nonrecurring Basis, Reconciliation, Calculation [Roll Forward] |  |  |  |
Impairment of intangible assets | $ (350.0) | $ (115.0) | (350.0) |
BIIB111 | Fair Value, Nonrecurring |  |  |  |
Fair Value, Assets Measured on Nonrecurring Basis, Reconciliation, Calculation [Roll Forward] |  |  |  |
Indefinite-lived Intangible Assets (Excluding Goodwill), Fair Value Disclosure |  |  | 365.0 |
Impairment of intangible assets |  |  | (350.0) |
Ending Book Value | 15.0 | 365.0 | 15.0 |
BIIB112 | Nightstar |  |  |  |
Fair Value, Assets Measured on Nonrecurring Basis, Reconciliation, Calculation [Roll Forward] |  |  |  |
Impairment of intangible assets | (191.6) |  | (191.6) |
BIIB112 | Fair Value, Nonrecurring |  |  |  |
Fair Value, Assets Measured on Nonrecurring Basis, Reconciliation, Calculation [Roll Forward] |  |  |  |
Indefinite-lived Intangible Assets (Excluding Goodwill), Fair Value Disclosure |  |  | 220.0 |
Impairment of intangible assets |  |  | (191.6) |
Ending Book Value | $ 28.4 | $ 220.0 | $ 28.4 |

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- Definition Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
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