Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Tax loss carryforwards	$ 38,494	$ 35,019
Research and development credits and other tax credits	3,710	3,595
Stock-based compensation	531	1,301
Other	1,252	1,126
Total deferred tax assets	43,987	41,041
Deferred tax liabilities:
Operating lease right-of-use assets	(154)	0
Other	(12)	0
Total deferred tax liabilities	166	0
Net deferred tax assets before valuation allowance	43,821	41,041
Valuation allowance	(43,821)	(41,041)
Net deferred tax asset	$ 0	$ 0