Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2019
Collaborative arrangements and non-collaborative arrangement transactions
Royalty cost of sales		$ 2,310.4	$ 2,533.4	$ 2,278.3
Income (loss) from equity method investments		0.0	0.0	2.6
ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Royalty cost of sales	$ 275.0
Income (loss) from equity method investments				111.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Amounts receivable		16.7	1.4
Amounts payable		$ 138.0	$ 118.4
Eisai | Milestone Payments
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs	45.00%				45.00%	45.00%
Eisai | Collaboration profit sharing
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense				224.7
Eisai | ADUHELM | Inventory, Idle Capacity Charges And Contractual Commitments
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense				$ 197.0