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Pension and Post-Retirement Plans (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Pension Benefits        
Components of net periodic (credit) expense:        
Interest cost $ 1,404 $ 1,470 $ 2,808 $ 2,940
Expected return on plan assets (4,073) (3,906) (8,146) (7,812)
Amortization of net actuarial loss 2,053 2,262 4,106 4,524
Net periodic (credit) expense (616) (174) (1,232) (348)
Post-Retirement Benefits        
Components of net periodic (credit) expense:        
Interest cost 144 160 288 320
Service cost   1   2
Amortization of prior-service costs (62) (62) (124) (124)
Amortization of net actuarial loss 68 81 136 162
Net periodic (credit) expense $ 150 $ 180 $ 300 $ 360