Income taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016
Income taxes [Abstract]
Income tax provision	$ 305	$ 429	$ 483	$ 888	$ 888
Effective tax rate	20.10%	33.10%	20.40%		32.90%
Total gross unrecognized tax benefits that would impact the effective rate	$ 104		$ 104
Previously unrecognized tax benefit recognized	28
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	$ 23		$ 23