Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended		3 Months Ended			6 Months Ended
	Feb. 29, 2020	Oct. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our condensed consolidated statements of income			$ 15.1	$ (16.3)		$ 0.4	$ (45.0)
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense Incurred By Collaboration			33.9	68.6		77.5	136.6
Expense reflected within statements of income			17.0	34.3		$ 38.7	68.3
Percentage of future development costs related to Eisai						45.00%
Loss on research and development contracts terminated with Eisai					$ 45.0
Additional Milestone Payment			75.0			$ 75.0
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our condensed consolidated statements of income			33.8			33.8
Sangamo Therapeutics, Inc. Agreement
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	$ 83.0
Skyhawk Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional Milestone Payment			2,400.0			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			35.5
Research and development | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			35.9	3.2		55.0	165.8
Expense reflected within statements of income			19.7	1.7		30.3	91.2
Research and development | UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			6.5			25.5
Expense reflected within statements of income			3.2			12.7
Selling, general and administrative | Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			1.4			6.3
Expense reflected within statements of income			0.7			3.2
Selling, general and administrative | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			45.2	0.2		67.9	21.2
Expense reflected within statements of income			$ 25.2	$ 0.1		$ 37.5	$ 11.7