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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 831 $ 776
Interest Cost 1,444 1,621
Expected Return on Plan Assets (2,255) (2,119)
Prior Service Cost Amortization 80 80
Actuarial Loss Amortization 1,618 1,081
Sub-total 1,718 1,439
Amounts Capitalized and Deferred (630) (412)
Net Periodic Benefit Cost Recognized 1,088 1,027
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 675 576
Interest Cost 780 856
Expected Return on Plan Assets (516) (411)
Prior Service Cost Amortization 303 303
Actuarial Loss Amortization 186 57
Sub-total 1,428 1,381
Amounts Capitalized and Deferred (532) (474)
Net Periodic Benefit Cost Recognized 896 907
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 71 60
Interest Cost 137 139
Prior Service Cost Amortization 14 3
Actuarial Loss Amortization 259 158
Sub-total 481 360
Amounts Capitalized and Deferred (138) (103)
Net Periodic Benefit Cost Recognized $ 343 $ 257