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Employee benefit plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Net periodic pension cost:        
Labor and Related Expense $ 6,946 $ 6,550 $ 21,291 $ 19,851
Pension Benefits        
Net periodic pension cost:        
Interest cost 233 217 389 433
Expected return on plan assets (360) (305) (600) (610)
Amortization of prior service cost 73 65    
Amortization of unrecognized net gain     122 130
Net periodic pension cost $ (54) $ (23) $ (89) $ (47)