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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 776 $ 848
Interest Cost 1,621 1,469
Expected Return on Plan Assets (2,119) (1,946)
Prior Service Cost Amortization 80 81
Actuarial Loss Amortization 1,081 1,447
Sub-total 1,439 1,899
Amounts Capitalized and Deferred (412) (720)
Net Periodic Benefit Cost Recognized 1,027 1,179
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 576 733
Interest Cost 856 851
Expected Return on Plan Assets (411) (409)
Prior Service Cost Amortization 303 327
Actuarial Loss Amortization 57 346
Sub-total 1,381 1,848
Amounts Capitalized and Deferred (474) (742)
Net Periodic Benefit Cost Recognized 907 1,106
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 60 122
Interest Cost 139 101
Prior Service Cost Amortization 3 47
Actuarial Loss Amortization 158 122
Sub-total 360 392
Amounts Capitalized and Deferred (103) (113)
Net Periodic Benefit Cost Recognized $ 257 $ 279