Collaborative and Other Relationships - Summary of Activity Related to ADUHELM Collaboration (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Collaborative arrangements and non-collaborative arrangement transactions
Selling, general and administrative	$ 788.2	$ 563.3	$ 1,941.2	$ 1,770.8
ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Aggregate gross idle capacity charges			$ 76.0
LEQEMBI Collaboration
Collaborative arrangements and non-collaborative arrangement transactions
Share of non-controling interest recognized	50.00%		50.00%
Selling, general and administrative	$ 38.7
Research and development | Eisai | ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration		34.6		116.0
Expense reflected within statements of income		19.0		63.8
Selling, general and administrative | Eisai | ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration		(8.0)		127.8
Expense reflected within statements of income		$ (3.9)		$ 68.1