Income Taxes - Effective Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective income tax rate reconciliation
Tax benefit at U.S. federal statutory rate	$ (17,629)	$ (6,890)	$ (10,592)
Stock based compensation	(47)	(3,415)	138
Effect of tax reform, change in federal tax rate			9,260
Other nondeductible expenses	14		1
State income taxes benefit before valuation allowance, net of federal benefit	(6,613)	(5,460)	(704)
Increase in domestic valuation allowance	26,843	17,529	2,880
Research and development credit	(2,636)	(1,604)	(825)
Other adjustments	$ 68	$ (160)	$ (158)