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Benefit Plans - Weighted-Average Asset Allocation of Pension Plan Assets (Detail) - Pension [Member]
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Equities 100.00% 100.00%
Domestic equities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Equities 38.00% 37.00%
Foreign equities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Equities 11.00% 11.00%
Fixed income [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Equities 49.00% 50.00%
Real estate [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Equities 2.00% 2.00%
Cash [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Equities 0.00% 0.00%