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Employee benefit plans - Net Periodic Pension Cost (Details) - Pension Benefits - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Defined Benefit Plan Disclosure        
Interest cost $ 217 $ 166 $ 433 $ 499
Expected return on plan assets (305) (223) (610) (670)
Amortization of unrecognized net gain 65 52 130 156
Net periodic pension cost $ (23) $ (5) $ (47) $ (15)