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Employee benefit plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Net periodic pension benefit:    
Labor and Related Expense $ 7,856 $ 7,595
Pension Benefits    
Net periodic pension benefit:    
Labor and Related Expense 306 340
Interest cost 54 128
Expected return on plan assets (123) (217)
Amortization of unrecognized net gain 22 45
Net periodic benefit $ (47) $ (44)