Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Revenue recognized in the period from:
Probable milestone earned and paid			$ 0	$ 500,000
Deferred revenue	$ 145,852,000	$ 82,305,000	145,852,000	82,305,000	$ 80,802,000	$ 89,967,000
Revenue from contract with customer	13,281,000	10,928,000	37,407,000	40,030,000
Revenue recognized that was previously deferred			237,000	13,368,000
Milestone earned, included in accounts receivable			500,000	5,000,000
Milestone related
Revenue recognized in the period from:
Revenue from contract with customer	500,000
Clinical materials revenue
Revenue recognized in the period from:
Revenue from contract with customer		1,427,000		2,465,000
Upon shipment | Clinical materials revenue
Revenue recognized in the period from:
Revenue recognized that was previously deferred				335,000
Genentech | Regulatory milestones
Revenue recognized in the period from:
Revenue from contract with customer	5,000,000		5,000,000
Kadcyla | Royalty revenue
Revenue recognized in the period from:
Deferred revenue	$ 65,200,000		65,200,000
Takeda
Revenue recognized in the period from:
Revenue from contract with customer				10,900,000
Takeda | Milestone related
Revenue recognized in the period from:
Deferred revenue		1,000,000		1,000,000
Potential milestone payment		$ 5,000,000		5,000,000
Takeda | License Revenue
Revenue recognized in the period from:
Revenue from contract with customer				10,900,000
Revenue recognized that was previously deferred				5,900,000
Debiopharm and Fusion | License Revenue
Revenue recognized in the period from:
Revenue recognized that was previously deferred				750,000
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue			$ 237,000
Revenue recognized that was previously deferred				$ 1,300,000