Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of future development costs related to Eisai				45.00%	15.00%
Loss on research and development contracts terminated with Eisai	$ 48.0	$ 45.0
Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	168.7		$ 64.9	$ 305.3		$ 176.0
Expense reflected within statements of income	84.3		32.5	152.6		88.0
Skyhawk Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional milestone payment to Skyhawk	2,000.0			2,000.0
Upfront and milestone payments made to collaborative partner		74.0
Research and development expense		38.5
Prepaid research and development expenses		$ 35.5
Research and development | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	4.7		64.8	170.5		204.8
Expense reflected within statements of income	2.6		55.1	93.8		183.6
Selling, general and administrative | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	0.1		12.1	21.3		33.3
Expense reflected within statements of income	$ 0.0		$ 5.1	$ 11.7		$ 19.0