Income taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Income taxes
Income tax expense (benefit)	$ 155	$ (173)	$ 303	$ (640)
Effective tax rate (as a percent)			(3.60%)	34.50%
Deferred tax liabilities	2,407		2,407
Uncertain tax positions	455		455		445
Uncertain tax positions, reduction to deferred tax assets	106		106		106
Accrued interest, net of federal income tax benefits, and penalties	63		63		53
Unrecognized tax benefits that would affect the effective tax rate	$ 286		$ 286