Income taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 43,814	$ 35,623
Tax Credits	5,518	4,678
Depreciation		86
Amortization	928	792
Stock-based compensation	973	541
Deferred rent	727
Lease incentive obligation	1,828
Deferred revenue	1,247	1,614
Other deferred tax assets	1,325	1,378
Total deferred tax assets	56,360	44,712
Valuation allowance	(54,137)	(44,033)
Net Deferred tax assets	2,223	679
Section 481(a) Adjustment - Accrued Bonus		(59)
Depreciation	(1,769)
Amortization of acquired intangibles	(3,031)	(610)
Inventory	(212)
Goodwill	(31)	(17)
Other deferred tax liabilities	(5)
Net deferred tax assets (liability)	$ 2,825	$ 7