Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($)	1 Months Ended				12 Months Ended
	Dec. 31, 2019	Mar. 31, 2018	Aug. 31, 2017	Mar. 31, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2016	Dec. 31, 2000
Revenue recognized in the period from:
Revenue from contract with customer					$ 132,299,000	$ 82,271,000	$ 53,446,000
Deferred revenue	$ 127,432,000				110,109,000	127,432,000	80,802,000
Contract assets	3,631,000					3,631,000	500,000
Contract liability	309,000				29,249,000	309,000
Contract Liabilities (deferred revenue), Impact Of Netting	(4,369,000)				4,369,000	(4,369,000)
Revenue recognized, previously deferred					61,872,000	14,817,000	14,139,000
Milestone earned, included in accounts receivable					(8,000,000)	(500,000)
Probable milestone earned and paid						(8,000,000)
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer					63,742,000	34,788,000	15,280,000
Upfront payment
Revenue recognized in the period from:
Contract liability					1,400,000
Total Revenue Recognized From Both The Beginning Balance And Current Period Increase In Contract Liability					3,200,000
Clinical materials revenue
Revenue recognized in the period from:
Revenue from contract with customer							4,635,000
Upon shipment | Clinical materials revenue
Revenue recognized in the period from:
Revenue recognized, previously deferred							300,000
CytomX and Novartis | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer						7,700,000
CytomX and Novartis | Technological Improvements
Revenue recognized in the period from:
Deferred revenue	300,000					300,000
CytomX and Novartis | Probable Milestone
Revenue recognized in the period from:
Contract assets	3,600,000					3,600,000
Contract Liabilities (deferred revenue), Impact Of Netting	(4,400,000)					(4,400,000)
Roche | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer										$ 2,000,000
Genentech | Regulatory milestones
Revenue recognized in the period from:
Revenue from contract with customer						5,000,000
CytomX
Revenue recognized in the period from:
Deferred revenue	200,000					200,000			$ 13,000,000
CytomX | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	7,300,000							$ 12,700,000
Deferred revenue					200,000
CytomX | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer					7,500,000
CytomX | Probable Milestone
Revenue recognized in the period from:
Contract assets					3,600,000
Contract liability					4,400,000
Jazz
Revenue recognized in the period from:
Revenue recognized, previously deferred						14,500,000
Jazz | License and milestone fees
Revenue recognized in the period from:
Amortization of deferred revenue					60,500,000
Jazz | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer			$ 75,000,000
Kadcyla | Royalty revenue
Revenue recognized in the period from:
Deferred revenue	$ 65,200,000					65,200,000
Takeda
Revenue recognized in the period from:
Deferred revenue, Deductions							(5,900,000)
Takeda | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer		$ 10,900,000			900,000				$ 8,600,000
Takeda | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer				$ 20,000,000
Amgen/Oxford BioTherapeutics
Revenue recognized in the period from:
Revenue from contract with customer					4,000,000
Amgen/Oxford BioTherapeutics | License and milestone fees
Revenue recognized in the period from:
Milestone earned, included in accounts receivable							1,000,000
Amgen/Oxford BioTherapeutics | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer										$ 1,000,000
Fusion Pharmaceuticals | Probable Milestone | Milestone related
Revenue recognized in the period from:
Revenue from contract with customer						500,000
Debiopharm and Another Collaborators
Revenue recognized in the period from:
Revenue recognized, previously deferred							800,000
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue						$ 300,000	2,100,000
Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd | Upfront payment
Revenue recognized in the period from:
Deferred revenue					40,000,000.0
Revenue recognized, previously deferred					$ 0
Right-to-test agreement | Takeda
Revenue recognized in the period from:
Revenue recognized, previously deferred							10,900,000
Adjustments due to new guidance | ASU 2014-09
Revenue recognized in the period from:
Revenue from contract with customer							$ 5,000,000.0