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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 824 $ 851
Interest Cost 1,514 1,486
Expected Return on Plan Assets (1,819) (1,814)
Prior Service Cost Amortization 66 66
Actuarial Loss Amortization 1,176 1,099
Sub-total 1,761 1,688
Amounts Capitalized and Deferred (662) (610)
Net Periodic Benefit Cost Recognized 1,099 1,078
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 744 652
Interest Cost 978 808
Expected Return on Plan Assets (337) (301)
Prior Service Cost Amortization 350 372
Actuarial Loss Amortization 524 262
Sub-total 2,259 1,793
Amounts Capitalized and Deferred (1,037) (737)
Net Periodic Benefit Cost Recognized 1,222 1,056
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 115 41
Interest Cost 98 96
Prior Service Cost Amortization 47 47
Actuarial Loss Amortization 74 94
Sub-total 334 278
Net Periodic Benefit Cost Recognized $ 334 $ 278