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Retirement plans - Pension Costs (Detail) - Retirement Plans - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Service cost-benefits earned during the period $ 8 $ 12 $ 872
Interest cost on benefit obligation 23,066 21,337 23,985
Expected return on plan assets (26,320) (30,935) (26,322)
Amortization of prior service cost 90 168 635
Amortization of actuarial loss 6,123 5,124 8,357
Pension payment timing related charge 686 7,498 26
Expense for company-sponsored retirement plans $ 3,653 $ 3,204 $ 7,553