Collaborative and Other Relationships - Collaborations (Details) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended			3 Months Ended				9 Months Ended
	Feb. 29, 2020 USD ($) $ / shares shares	Oct. 31, 2019 USD ($)	Jan. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) product $ / shares	Sep. 30, 2019 USD ($)	Jan. 31, 2020 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Loss on equity method investment								$ 14.7	$ 63.5
Unrealized Gain (Loss) on Investments				$ 82.1		$ (40.3)
Ionis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Unrealized Gain (Loss) on Investments								(56.7)
Denali Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Unrealized Gain (Loss) on Investments								(29.6)
Sangamo Therapeutics, Inc. Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Unrealized Gain (Loss) on Investments								$ (56.1)
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of product candidates | product								2
Expense Incurred By Collaboration				75.7		168.7		$ 153.2	305.3
Expense reflected within statements of income				37.9		84.3		$ 76.6	152.6
Percentage of future development costs related to Eisai								45.00%
Loss on research and development contracts terminated with Eisai							$ 45.0
Loss on equity method investment				0.0		0.0		$ 33.8	0.0
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 | shares	24
Global Licensing Collaboration Agreement, Shares Purchased Per Share | $ / shares	$ 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]
Upfront and milestone payments made to collaborative partner		$ 15.0				74.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments			$ 2,400.0
Research and development expense						38.5
Prepaid research and development expenses						35.5			35.5
Denali Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global Licensing Collaboration Agreement, Amount, Shares Purchased								$ 465.0
Global Licensing Collaboration Agreement, Shares Purchased Per Share | $ / shares								$ 34.94
Upfront and milestone payments made to collaborative partner								$ 560.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments								1,100.0
Research and development expense								$ 41.3
Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront and milestone payments made to collaborative partner					$ 100.0
Collaboration agreement term								5 years
Research and development expense					$ 63.0
Prepaid research and development expenses										$ 37.0
Research and development | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				37.5		4.7		$ 92.5	170.5
Expense reflected within statements of income				20.6		2.6		50.9	93.8
Research and development | UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				10.2				35.7
Expense reflected within statements of income				5.2				17.9
Selling, general and administrative | Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						10.2		6.3	24.8
Expense reflected within statements of income						5.1		3.2	12.4
Selling, general and administrative | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				90.9		0.1		158.8	21.3
Expense reflected within statements of income				$ 51.3		$ 0.0		$ 88.8	$ 11.7