Collaborative and Other Relationships - Other Arrangements (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended					12 Months Ended
	Aug. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Apr. 30, 2020	Feb. 29, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Collaborative arrangements and non-collaborative arrangement transactions
Research and development						$ 2,501.2	$ 3,990.9	$ 2,280.6
Sage Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount		$ 650.0
Global licensing collaboration, shares purchased (in shares)		6.2
Global licensing collaboration, purchase price per share (in dollars per share)		$ 104.14
Research and development expense asset acquired		$ 209.0
Upfront and milestone payments made to collaborative partner		875.0
Global licensing collaboration, development and commercial milestone payments		$ 1,600.0
Denali Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount			$ 465.0
Global licensing collaboration, shares purchased (in shares)			13.0
Global licensing collaboration, purchase price per share (in dollars per share)			$ 34.94
Research and development expense asset acquired			$ 41.3
Upfront and milestone payments made to collaborative partner			560.0
Global licensing collaboration, development and commercial milestone payments			$ 1,100.0
Sangamo Therapeutics, Inc. Agreement
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount				$ 225.0
Global licensing collaboration, shares purchased (in shares)				24.0
Global licensing collaboration, purchase price per share (in dollars per share)				$ 9.21
Research and development expense asset acquired					$ 83.0
Upfront and milestone payments made to collaborative partner					125.0
Global licensing collaboration, development and commercial milestone payments					$ 2,400.0
Term of collaboration agreement					5 years
Investment in common stock, shares purchased (in shares)						12.0
Global licensing collaboration, payment, selection of targets					$ 80.0
Global licensing collaboration, payment, first milestone					1,900.0
Global licensing collaboration, payment, sales based milestone					$ 380.0
Other research and discovery
Collaborative arrangements and non-collaborative arrangement transactions
Research and development						$ 89.1	$ 92.1	$ 80.8
InnoCare Pharma Limited (InnoCare) Agreement
Collaborative arrangements and non-collaborative arrangement transactions
Upfront and milestone payments made to collaborative partner	$ 125.0
Contingent milestone payments made to collaborative partner	$ 812.5