Fair Value, Assets Measured on Nonrecurring Basis, Reconciliation, Calculation

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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

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- Definition

Fair value portion of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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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