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Employee benefit plans - Components of Net Periodic Benefit Cost (Detail) - Pension Benefits - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Net periodic pension cost:        
Interest cost $ 156 $ 217 $ 156 $ 217
Expected return on plan assets (240) (305) (240) (305)
Amortization of prior service cost 49 65    
Amortization of unrecognized net gain     49 65
Net periodic pension cost $ (35) $ (23) $ (35) $ (23)