Summary of Significant Accounting Policies - Subsequent Events (Details) - USD ($) $ in Thousands, shares in Millions			3 Months Ended												12 Months Ended
	Feb. 21, 2020	Jan. 27, 2020	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
Subsequent Event [Line Items]
Net proceeds generated from public offering																$ 162,512	$ 101,663
Revenue from contract with customer			$ 44,864	$ 13,281	$ 15,542	$ 8,584	$ 13,416	$ 10,928	$ 9,287	$ 19,815	$ 39,448	$ 8,480	$ 39,020	$ 28,499	$ 82,271	$ 53,446	$ 115,447
Subsequent event
Subsequent Event [Line Items]
Issuance of common stock (in shares)		24.5
Net proceeds generated from public offering		$ 97,700
Development milestones | Subsequent event | CytomX
Subsequent Event [Line Items]
Revenue from contract with customer	$ 3,000