Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Revenue recognized in the period from:
Revenue from contract with customer	$ 15,025,000	$ 15,542,000	$ 28,312,000	$ 24,126,000
Deferred revenue	126,615,000	145,931,000	126,615,000	145,931,000	$ 127,432,000	$ 80,802,000
Contract assets	1,042,000		1,042,000		3,631,000	$ 500,000
Contract liability	80,000		80,000		309,000
Contract Liabilities, Impact Of Netting	411,000		411,000
Revenue previously deferred	945,000	79,000	1,228,000	158,000
Milestone earned, included in accounts receivable			3,000,000	500,000
Probable milestone earned and paid				0
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	945,000	5,079,000	1,228,000	5,158,000
Genentech | Regulatory milestones
Revenue recognized in the period from:
Revenue previously deferred		5,000,000.0		5,000,000.0
CytomX | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer			200,000
CytomX | Probable Milestone
Revenue recognized in the period from:
Contract assets					2,700,000
Contract liability					$ 300,000
Kadcyla | Royalty revenue
Revenue recognized in the period from:
Deferred revenue		65,200,000		65,200,000
Takeda | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	870,000		870,000
Takeda | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue	$ 870,000		870,000
Fusion Pharmaceuticals | Probable Milestone
Revenue recognized in the period from:
Contract assets		$ 500,000		500,000
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue			$ 1,000,000.0	$ 158,000