Income taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Income taxes
Income tax expense	$ 141	$ 124	$ 340	$ 362
Effective tax rate (as a percent)	1.80%	1.70%	2.40%	2.20%
Uncertain tax positions	$ 387		$ 387		$ 380
Uncertain tax positions, reduction to deferred tax assets	84		84		84
Accrued interest, net of federal income tax benefits, and penalties	74		74		$ 67
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	$ 229		$ 229