Income taxes - Components of the net deferred tax assets or liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 29,180	$ 19,011
Credit carryforwards	180
Accrued expenses and other	669	94
Stock-based compensation	2,717	51
Operating lease liabilities	1,740
Other liabilities	132
Total deferred tax assets	34,618	19,156
Valuation allowance	(32,583)	$ (19,156)	$ (13,789)	$ (8,378)
Total deferred tax assets	2,035
Deferred tax liabilities:
Operating lease right of use asset	(1,808)
Finance lease right of use asset	(227)
Total deferred tax liabilities	$ (2,035)