Income taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Income taxes
Income tax expense	$ 204	$ 148
Effective tax rate (as a percent)	2.70%	2.90%
Deferred tax liabilities	2,767
Uncertain tax positions	465		459
Uncertain tax positions, reduction to deferred tax assets	106		106
Accrued interest, net of federal income tax benefits, and penalties	73		67
Unrecognized tax benefits that would affect the effective tax rate	$ 286