Income taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Income taxes
Income tax expense	$ 95	$ 115	$ 457	$ 401
Effective tax rate (as a percent)	1.80%	2.40%	2.10%	2.20%
Uncertain tax positions	$ 376		$ 376		$ 363
Uncertain tax positions, reduction to deferred tax assets	84		84		84
Accrued interest, net of federal income tax benefits, and penalties	63		63		$ 50
Unrecognized tax benefits that would affect the effective tax rate	$ 229		$ 229