Income taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income taxes
Income tax expense	$ 167	$ 95	$ 507	$ 457
Effective tax rate (as a percent)	5.40%	1.80%	2.90%	2.10%
Uncertain tax positions	$ 389		$ 389		$ 380
Uncertain tax positions, reduction to deferred tax assets	84		84		84
Accrued interest, net of federal income tax benefits, and penalties	76		76		$ 67
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	$ 229		$ 229