Income taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income taxes
Reduction of federal income tax rate	21.00%	21.00%
Deferred Tax Assets:
Net operating loss	$ 37,747	$ 37,819
Share-based compensation	2,113	1,757
Research and development tax credit	1,947	2,378
Other reserves	2,238	1,270
Finance lease liability	156	390
Operating lease liability	826	208
State deferred	1,518	1,236
Inventory	90	103
Accrued rebates	5,862	4,902
Interest expense carryover	1,143	1,296
Other	914	952
Total deferred tax assets	54,554	52,311
Deferred Tax Liabilities:
Intangible assets	(1,230)	(1,226)
Right-of-use asset	(639)
Property and equipment	(873)	(999)
Total deferred tax liabilities	(2,742)	(2,225)
Valuation allowance	$ 51,812	$ 50,086