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Employee benefit plans - Additional Information (Details 2) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Compensation and Retirement Disclosure [Abstract]        
Service cost $ 174 $ 169 $ 522 $ 507
Interest cost (233) (227) (699) (681)
Amortization of unrecognized net loss 50 23 150 69
Net periodic pension cost $ (9) $ (35) $ (27) $ (105)