Income Taxes - Deferred Income Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Components of Deferred Tax Assets [Abstract]
Net operating loss carryforwards	$ 83,671	$ 65,487
Research and development tax credit carryforwards	4,616	3,654
Capitalized research and development expenses	2,193	2,626
Lease liabilities	1,440	765
Other	2,452	1,413
Total deferred tax assets	94,372	73,945
Valuation allowance	(92,197)	(73,410)
Deferred tax assets, net of valuation allowance	2,175	535
Right of use assets	2,175	535
Total deferred tax liabilities	2,175	535
Total deferred tax assets, net	$ 0	$ 0