Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 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		$ 45.0
Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	$ 68.6		$ 54.6	$ 136.6	$ 111.1
Expense reflected within statements of income	34.3		27.3	68.3	55.5
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	3.2		76.5	165.8	140.0
Expense reflected within statements of income	1.7		65.0	91.2	128.5
Selling, general and administrative | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	0.2		14.0	21.2	21.2
Expense reflected within statements of income	$ 0.1		$ 9.2	$ 11.7	$ 13.9