Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 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				$ 36.8	$ 305.6
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
ADUHELM | Cost of Sales
Collaborative arrangements and non-collaborative arrangement transactions
Excess and obsolescence charges related to inventory			$ 45.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs related to Eisai			45.00%
Amounts receivable	$ 23.5			23.5		$ 88.0
Amounts payable	99.3			99.3		$ 81.2
Eisai | Collaboration profit sharing
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense	$ 28.9			$ 210.6
Eisai | ADUHELM | Inventory, Idle Capacity Charges And Contractual Commitments
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense					$ 160.0