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Commitments and Contingencies - Narrative (Details) - USD ($)	1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2019	Oct. 31, 2019	Dec. 31, 2012	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2021	Dec. 31, 2013
Business Acquisition [Line Items]
Maximum contingent consideration in the form of development and approval milestones					$ 400,000,000.0
Contingent payment				$ 300,000,000.0	0	$ 0	$ 58,200,000
Potential future milestone payments commitment, approximately					10,200,000,000
Accrued expenses					21,700,000
Cancellable future commitments					593,000,000.0
Liabilities associated with uncertain tax positions					79,600,000
Income taxes paid					62,000,000.0
Solothurn
Business Acquisition [Line Items]
Contractual obligation					9,300,000
2017 Tax Act
Business Acquisition [Line Items]
Transition toll tax liabilities	$ 697,000,000.0				697,000,000.0	697,000,000.0
Twelve months
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					200,000,000.0
Development Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					1,900,000,000
Regulatory Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					1,300,000,000
Commercial Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					$ 7,000,000,000.0
Ionis Pharmaceuticals
Business Acquisition [Line Items]
Total aducanumab collaboration third party milestone expense	155,000,000.0		$ 10,000,000.0
Ionis Pharmaceuticals | Minimum | SPINRAZA
Business Acquisition [Line Items]
Percentage of royalties as per collaboration					11.00%			11.00%
Ionis Pharmaceuticals | Maximum | SPINRAZA
Business Acquisition [Line Items]
Percentage of royalties as per collaboration					15.00%			15.00%
Alkermes
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately	$ 155,000,000.0					$ 155,000,000.0
Alkermes | VUMERITY
Business Acquisition [Line Items]
Percentage of royalties as per collaboration		15.00%			15.00%
Royalties payable period, post FDA approval					5 years
Aducanumab | Forecast
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately									$ 86,200,000
Aducanumab | Regulatory Milestones | European Union
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					$ 50,000,000.0
Aducanumab | Regulatory Milestones | JAPAN
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					50,000,000.0
Neurimmune | Regulatory Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					100,000,000.0
TYSABRI
Business Acquisition [Line Items]
Future contingent payment for annual worldwide net sales up to $2.0 billion										18.00%
Future contingent payment threshold										$ 2,000,000,000.0
Future contingent payment for annual worldwide net sales that exceed $2.0 billion										25.00%
Fumapharm AG | Fumarate | Twenty billion
Business Acquisition [Line Items]
Cumulative sales level							20,000,000,000.0
Fumapharm AG | Fumarate | Each additional one billion up to twenty billion
Business Acquisition [Line Items]
Cumulative sales level					$ 20,000,000,000.0		$ 20,000,000,000.0

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