Income Taxes - Components of deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating losses	$ 64,259	$ 40,554
Tax credit carryforwards	5,670	2,448
Accrued expenses	4,058	2,520
Lease liabilities	3,166	791
Licensed technology	1,534	1,402
Deferred revenue	1,088	1,315
Depreciation	502	492
Other	84	77
Total gross deferred tax assets	80,361	49,599
Valuation allowance	(77,375)	(48,889)
Net deferred tax assets less valuation allowance	2,986	710
Deferred tax liabilities
Right-of-use assets	(2,986)	(710)
Total gross deferred tax liabilities	(2,986)	(710)
Net deferred taxes	$ 0	$ 0