Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Revenue recognized in the period from:
Revenue from contract with customer	$ 18,189	$ 13,281	$ 46,501	$ 37,407
Deferred revenue	130,526	145,852	130,526	145,852	$ 127,432	$ 80,802
Contract assets					3,631	$ 500
Contract liability	3,985		3,985		309
Contract Liabilities, Impact Of Netting	4,369		4,369
Revenue previously deferred	72	79	1,300	237
Milestone earned, included in accounts receivable			8,000	500
Probable milestone earned and paid				0
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	$ 97	79	1,325	5,237
Genentech | Regulatory milestones
Revenue recognized in the period from:
Revenue previously deferred				5,000
CytomX | License and milestone fees
Revenue recognized in the period from:
Revenue previously deferred			200
CytomX | Probable Milestone
Revenue recognized in the period from:
Contract assets					3,600
Contract liability					$ 4,400
Kadcyla | Royalty revenue
Revenue recognized in the period from:
Deferred revenue		65,200		65,200
Takeda | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue			900
Fusion Pharmaceuticals | Probable Milestone
Revenue recognized in the period from:
Contract assets		$ 500		500
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue			$ 1,100	$ 200