Income taxes, Components of income tax provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current [Abstract]
Federal	$ 2,379	$ 1,776	$ 450
State	114	70	22
Foreign	(49)	2	0
Income taxes current	2,444	1,848	472
Deferred [Abstract]
Federal	1,097	(257)	843
State	20	(38)	35
Foreign	0	0	0
Income taxes deferred	1,117	(295)	878
Income tax (benefit) provision	3,561	1,553	1,350
Income taxes [Abstract]
Foreign loss before taxes	563	$ 235	$ (174)
Federal [Member]
Income taxes [Abstract]
Net operating loss carryforward	0
State [Member]
Income taxes [Abstract]
Net operating loss carryforward	0
Tax credit carryforward	0
R&D Credit [Member]
Income taxes [Abstract]
Tax credit carryforward	$ 0