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Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2022	Mar. 31, 2022	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Collaborative arrangements and non-collaborative arrangement transactions
Income (loss) from equity method investments					$ 2.6	$ 34.9	$ 3.3
Royalty cost of sales					2,278.3	2,109.7	1,805.2
Regulatory Milestones | Neurimmune
Collaborative arrangements and non-collaborative arrangement transactions
Upfront and milestone payments made to collaborative partner						100.0	75.0
ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Research and development expense asset acquired					335.0
Income (loss) from equity method investments					111.0	30.0
Royalty cost of sales	$ 164.0	$ 275.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Additional milestone payment					0.0	100.0	75.0
Income (loss) from equity method investments				$ 33.8	0.0	45.0	33.8
Amounts receivable	88.0				88.0	285.4
Amounts payable	$ 81.2				$ 81.2	46.5
Eisai | Milestone Payments
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs					45.00%
Eisai | Collaboration profit sharing
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense					$ 224.7	233.2
Eisai | ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Income (loss) from equity method investments			$ 45.0			59.0
Eisai | ADUHELM | Milestone Payments
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense						45.0	$ 33.8
Eisai | ADUHELM | Inventory, Idle Capacity Charges And Contractual Commitments
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense					$ 197.0	$ 99.0

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