Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs related to Eisai				45.00%
Excess and obsolescence charges related to inventory				$ 62.1	$ 321.6
ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Excess and obsolescence charges related to inventory		$ 275.0	$ 275.0
ADUHELM | Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Research and development expense asset acquired				335.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs related to Eisai		45.00%
Amounts receivable	$ 34.3			34.3		$ 88.0
Amounts payable	122.6			122.6		$ 81.2
Eisai | Collaboration profit sharing
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense	$ 3.4			$ 214.0
Eisai | ADUHELM | Inventory, Idle Capacity Charges And Contractual Commitments
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense					$ 177.0