Income Taxes - Deferred tax assets net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 42,486	$ 31,533	$ 15,438
Tax credit carryforwards	6,990	6,441	4,351
Capitalized tax assets		138	131
Capitalized tax assets	(3)
Accruals	152	188	183
Stock compensation	4,317	2,879	1,730
Operating lease liability	892
Other	40	58	52
Total deferred tax assets	54,874	41,237	21,885
Deferred tax liabilities
Operating lease right-of-use asset	(651)
Valuation allowance	(54,223)	(41,237)	(21,885)
Net deferred tax assets	0	0	$ 0
Increase in valuation allowance	$ 13,000	$ 19,400