Income Taxes - Reconciliation of Tax Expense Computed at Statutory Federal Rate and Company's Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory federal rate	$ (11,446)	$ (10,570)	$ (10,143)
Stock-based compensation	(5,560)	(8,557)	(7,634)
Meals and Entertainment	409	309	198
Other	614	148	(145)
Tax credits	(1,128)	(1,015)	(644)
2017 Tax Act	0	44	21,969
Change in valuation allowance	17,176	19,685	(3,601)
Total Tax Expense (benefit)	$ 65	$ 44	$ 0