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Benefit Plans - Target Allocation of Assets and Acceptable Ranges (Detail) - Pension [Member]
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 100.00%
Domestic equities [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 37.00%
Foreign equities [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 11.00%
Fixed Income [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 50.00%
Real estate [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 2.00%
Cash [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 0.00%
Minimum [Member] | Domestic equities [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 30.00%
Minimum [Member] | Foreign equities [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 5.00%
Minimum [Member] | Fixed Income [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 40.00%
Minimum [Member] | Real estate [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 0.00%
Minimum [Member] | Cash [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 0.00%
Maximum [Member] | Domestic equities [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 41.00%
Maximum [Member] | Foreign equities [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 13.00%
Maximum [Member] | Fixed Income [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 65.00%
Maximum [Member] | Real estate [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 4.00%
Maximum [Member] | Cash [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of assets 0.00%