Income taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Income taxes
Income tax expense	$ 199	$ 238
Effective tax rate (as a percent)	3.00%	2.50%
Uncertain tax positions	$ 384		$ 380
Uncertain tax positions, reduction to deferred tax assets	84		84
Accrued interest, net of federal income tax benefits, and penalties	71		$ 67
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	$ 229