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Employee Benefit Plans - Summary of Weighted-Average Assets Allocation (Detail) - Pension Plan [Member]
Dec. 31, 2017
Dec. 31, 2016
Weighted-Average Assets Allocation    
Weighted-Average Assets Allocation 100.00% 100.00%
Equity Securities [Member]    
Weighted-Average Assets Allocation    
Weighted-Average Assets Allocation   28.92%
Fixed Income [Member]    
Weighted-Average Assets Allocation    
Weighted-Average Assets Allocation 65.40% 71.08%
Cash [Member]    
Weighted-Average Assets Allocation    
Weighted-Average Assets Allocation 34.60%