Revenue from Contracts with Customers - Additional Information (Details)		1 Months Ended		3 Months Ended		9 Months Ended
	Oct. 27, 2020 USD ($) $ / shares shares	Oct. 29, 2020 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) agreement	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Revenue from Contracts with Customers [Line Items]
Bad debt expense						$ 0
Number of agreements entered into | agreement			2
Collaboration and license agreement revenues				$ 1,061,731,000	$ 213,263,000	1,574,243,000	$ 626,909,000
Fair value of shares issued					548,691,000
License and Service [Member]
Revenue from Contracts with Customers [Line Items]
Collaboration and license agreement revenues				758,313,000	18,420,000	780,250,000	99,128,000
Merck [Member] | License and Service [Member]
Revenue from Contracts with Customers [Line Items]
Collaboration and license agreement revenues				725,000,000	$ 0	725,000,000	0
Subsequent Event [Member]
Revenue from Contracts with Customers [Line Items]
Shares issued (in shares) | shares	5,000,000	5,000,000
Price per share (in dollars per share) | $ / shares	$ 200	$ 200
Proceeds from sale of stock	$ 1,000,000,000.0	$ 1,000,000,000.0
Fair value of shares issued	749,900,000	$ 749,900,000
Purchase Agreement [Member] | Subsequent Event [Member] | License and Service [Member]
Revenue from Contracts with Customers [Line Items]
Collaboration and license agreement revenues	$ 250,100,000
Collaboration and License Agreement Revenues [Member]
Revenue from Contracts with Customers [Line Items]
Deferred revenue recognized						0.0	27,500,000
Collaboration and License Agreement Revenues [Member] | Takeda Collaboration and License Agreement [Member]
Revenue from Contracts with Customers [Line Items]
Milestone payment revenue							$ 37,500,000
Collaboration and License Agreement Revenues [Member] | LV Agreement [Member]
Revenue from Contracts with Customers [Line Items]
Upfront cash payment received			$ 600,000,000.0
Milestone payment (up to)			850,000,000.0
Milestone payment upon achievement of annual sales threshold (up to)			$ 1,750,000,000
Percentage of costs entity is responsible for			50.00%
Percentage of profits to be received			50.00%
Collaboration and License Agreement Revenues [Member] | LV Agreement [Member] | Merck [Member] | License and Service [Member]
Revenue from Contracts with Customers [Line Items]
Collaboration and license agreement revenues				600,000,000.0		600,000,000.0
Collaboration and License Agreement Revenues [Member] | TUKYSA Agreement [Member]
Revenue from Contracts with Customers [Line Items]
Upfront cash payment received			$ 125,000,000.0
Milestone payment (up to)			65,000,000.0
Prepaid research and development expense			85,000,000.0
Remaining co-development liability			84,500,000	84,500,000		84,500,000
Collaboration and License Agreement Revenues [Member] | Purchase Agreement [Member]
Revenue from Contracts with Customers [Line Items]
Variable consideration			$ 250,100,000	$ 250,100,000		$ 250,100,000