Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 61,160	$ 25,508
Research and development tax credit carryforwards	13,525	3,824
Accrued expenses	1,142	1,139
Capitalized intellectual property costs	1,073	764
Capitalized research and development expense	109	120
Operating lease liabilities	12,952
Stock-based compensation expense	9,156	2,652
Total deferred tax assets	99,117	34,007
Deferred tax liabilities:
Operating lease assets	(12,720)
Depreciation and other	(513)	(341)
Total deferred tax liabilities	(13,233)	(341)
Valuation allowance	$ (85,884)	$ (33,666)	$ (12,575)