Income taxes - Deferred Taxes (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 21, 2017	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Income taxes
Reduction of federal income tax rate	35.00%	21.00%	21.00%	34.00%
Reduction of deferred tax expense				$ 20,600
Deferred Tax Assets:
Net operating loss		$ 37,818	$ 37,818	32,193
Share-based compensation		1,757	1,757	1,340
R&D tax credit		2,378	2,378	1,696
Other reserves		1,616	1,616	1,069
Capital lease liability		390	390	563
State deferreds		1,236	1,236	397
Inventory		103	103	829
Accrued rebates		4,902	4,902	561
Other current assets				319
Other		2,456	2,456	2,597
Total deferred tax assets		52,656	52,656	41,564
Deferred Tax Liabilities:
Intangible assets		(1,571)	(1,571)	(1,846)
Property and equipment		(999)	(999)	(747)
Total deferred tax liabilities		(2,570)	(2,570)	(2,593)
Valuation allowance		$ (50,086)	$ (50,086)	$ (38,971)