Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax at the statutory federal rate	$ (14,642)	$ (17,690)	$ (28,293)
Other nondeductible items	887	329	371
Stock-based compensation	(33,042)	497	3,819
Research and development credits	(5,266)	(1,726)	(714)
Change in valuation allowance	59,049	22,516	5,415
State taxes, net of federal benefits	(8,253)	(4,231)	(1,868)
Change in tax rate due to Tax Act	0	0	21,346
Other	(605)	343	(69)
Total provision for income taxes	$ (1,872)	$ 38	$ 7