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Intangible Assets and Goodwill - Narrative (Details) - USD ($)	3 Months Ended			12 Months Ended
	Dec. 31, 2020	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Jun. 07, 2019
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets				$ 464,800,000	$ 489,900,000	$ 747,300,000
Amortization of acquired intangible assets				255,100,000	274,000,000.0	381,200,000
Accumulated impairment losses related to goodwill	$ 0			0
Intangible Assets, Net (Excluding Goodwill)	3,084,300,000			3,084,300,000	3,527,400,000
TECFIDERA
Goodwill [Line Items]
Impairment						176,800,000
In-process research and development
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets				19,300,000
In-process research and development
Goodwill [Line Items]
Impairment		$ 215,900,000	$ 189,300,000		215,900,000	$ 189,300,000
Intangible assets (excluding goodwill)	762,500,000			762,500,000	$ 965,500,000
BIIB111 | In-process research and development
Goodwill [Line Items]
Intangible Assets, Net (Excluding Goodwill)	365,000,000.0			365,000,000.0			$ 480,000,000.0
TGN | In-process research and development
Goodwill [Line Items]
Intangible assets (excluding goodwill)	177,500,000			177,500,000
Nightstar
Goodwill [Line Items]
Intangible assets								$ 585,000,000.0
Nightstar | BIIB111
Goodwill [Line Items]
Impairment				115,000,000.0
Intangible assets								$ 480,000,000.0
Nightstar | BIIB111 | In-process research and development
Goodwill [Line Items]
Impairment	115,000,000.0
Nightstar | BIIB054
Goodwill [Line Items]
Impairment	$ 75,400,000			$ 75,400,000

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Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date.

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Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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