Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022	Mar. 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	$ 17.4		$ 281.5
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	72.6	88.0
Amounts payable	137.8	$ 81.2
Eisai | Collaboration profit sharing
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense	$ 181.7
Eisai | ADUHELM | Inventory, Idle Capacity Charges And Contractual Commitments
Collaborative arrangements and non-collaborative arrangement transactions
Recognized net reductions to operating expense			$ 160.0