Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($)	1 Months Ended				3 Months Ended												12 Months Ended
	Dec. 31, 2019	Mar. 31, 2018	Aug. 31, 2017	Mar. 31, 2015	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	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					$ 44,864,000	$ 13,281,000	$ 15,542,000	$ 8,584,000	$ 13,416,000	$ 10,928,000	$ 9,287,000	$ 19,815,000	$ 39,448,000	$ 8,480,000	$ 39,020,000	$ 28,499,000	$ 82,271,000	$ 53,446,000	$ 115,447,000
Deferred revenue	$ 127,432,000				127,432,000				80,802,000				89,967,000				127,432,000	80,802,000	89,967,000
Contract asset	3,631,000				3,631,000				500,000								3,631,000	500,000
Contract Liabilities, Impact Of Netting	(4,369,000)				(4,369,000)				5,000,000								(4,369,000)	5,000,000
Revenue previously deferred																	14,817,000	14,139,000
Milestone earned, included in accounts receivable																	(500,000)	(5,000,000)
Probable milestone earned and paid																	(8,000,000)	500,000
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer					29,551,000	$ 79,000	$ 5,079,000	$ 79,000	1,747,000	672,000	1,321,000	11,540,000	29,580,000	79,000	31,080,000	18,730,000	34,788,000	15,280,000	79,469,000
Clinical materials revenue
Revenue recognized in the period from:
Revenue from contract with customer									$ 2,170,000	$ 1,427,000	$ 336,000	$ 702,000	$ 1,829,000	$ 1,248,000	$ 599,000	$ 678,000		4,635,000	4,354,000
Upon shipment | Clinical materials revenue
Revenue recognized in the period from:
Revenue previously deferred																		335,000
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												300,000
CytomX and Novartis | Probable Milestones
Revenue recognized in the period from:
Contract asset	3,600,000				3,600,000												3,600,000
Contract Liabilities, Impact Of Netting	(4,400,000)				(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												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				200,000												200,000
CytomX | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer																	7,500,000
Jazz
Revenue recognized in the period from:
Revenue previously deferred																	14,500,000
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												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																		$ 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 Milestones | 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 previously deferred																		750,000
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue																	$ 317,000	2,100,000
Right-to-test agreement | Takeda
Revenue recognized in the period from:
Revenue 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