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Pension and Post-Retirement Plans (Detail) - Pension Benefits - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Components of net periodic pension credit:      
Interest cost $ 5,616 $ 5,881 $ 6,063
Expected return on plan assets (16,290) (15,627) (17,559)
Amortization of net actuarial loss 8,209 9,050 8,208
Net periodic benefit cost $ (2,465) $ (696) $ (3,288)