Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Income Taxes
Net operating loss carryforwards	$ 67,368	$ 49,018
Lease liabilities	8,152	9,369
Stock-based compensation	7,413	5,414
Capitalized research and development and start-up expenditures	5,258	6,733
Research and development tax credit carryforwards	2,398	6,250
Rebates, incentives, trade discounts and allowances	2,177
Other	2,609	2,467
Total deferred tax assets	95,375	79,251
Right-of-use assets	(7,810)	(9,178)
Total deferred tax liabilities	(7,810)	(9,178)
Valuation allowance	$ (87,565)	$ (70,073)