Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended		3 Months Ended
	Feb. 29, 2020	Oct. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
BAN2401
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense				$ 68.0
Elenbecestat Collaboration
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense				34.0
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense Incurred By Collaboration			$ 43.5
Expense reflected within statements of income			$ 21.8
Percentage of future development costs related to Eisai			45.00%
Loss on research and development contracts terminated with Eisai				45.0
UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense Incurred By Collaboration			$ 19.0
Sangamo Therapeutics, Inc. Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global Licensing Collaboration Agreement, Amount, Shares Purchased	$ 225.0
Global Licensing Collaboration Agreement, Shares, Purchased	24
Global Licensing Collaboration Agreement, Shares Purchased Per Share	$ 9.21
Upfront and milestone payments made to collaborative partner	$ 125.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments	2,400.0
Global Licensing Collaboration Agreement, Payment, Selection of Targets	80.0
Global Licensing Collaboration Agreement, Payment, Achievement of First Commercial Sale Milestones	1,900.0
Global Licensing Collaboration Agreement, Payment, Achievement of Specific Sales Based Milestones	380.0
Research and development expense	$ 85.0
Skyhawk Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional Milestone Payment			2,400.0
Upfront and milestone payments made to collaborative partner		$ 15.0		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			19.1	162.5
Expense reflected within statements of income			10.5	89.4
Research and development | UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense Incurred By Collaboration			9.5
Selling, general and administrative | Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			4.9
Expense reflected within statements of income			2.4
Selling, general and administrative | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			22.7	20.9
Expense reflected within statements of income			$ 12.3	$ 11.6