Income Taxes - Components of deferred tax assets (liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Components of deferred tax assets
Net operating loss carryforwards	$ 14,519	$ 10,228
Accruals and reserves	1,292	1,053
Research and development credits	1,534	1,368
Deferred Revenue		6
Depreciation and amortization	(34)	(23)
Total	17,311	12,632
Valuation allowance	(17,311)	(12,632)
Net change in the valuation allowance	$ 4,700	$ 5,800