Income Taxes - Reconciliation of Effective Tax Rate to the Statutory Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Amount
Income tax expense (benefit) at federal statutory rate	$ 1,818	$ (11,457)
State income tax, net of federal benefit	618	(1,762)
Non-deductible warrant expense	284	2,184
Other permanent differences	46	70
Research and development tax credits	(715)	(1,058)
Other tax credits	1	(934)
Tax reserve reassessment	25	196
Change in valuation allowance	(2,696)	12,856
Return adjustment	752	0
Stock-based compensation	279	96
Other, net	(3)	(22)
Total tax expense	$ 409	$ 169
Percent
Income tax expense (benefit) at federal statutory rate	21.00%	21.00%
State income tax, net of federal benefit	7.10%	3.20%
Non-deductible warrant expense	3.30%	(4.00%)
Other permanent differences	0.50%	(0.10%)
Research and development tax credits	(8.30%)	1.90%
Other tax credits	(0.00%)	1.70%
Tax reserve reassessment	0.30%	(0.40%)
Change in valuation allowance	(31.10%)	(23.50%)
Return adjustment	8.70%	0.00%
Stock-based compensation	3.20%	(0.20%)
Other, net	0.00%	0.10%
Income tax expense	4.70%	(0.30%)