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Retirement Plans - Summary of Plan's Weighted Average Asset Allocations (Detail)
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Total weighted average asset 100.00% 100.00%
Equity Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total weighted average asset 61.00% 61.00%
Debt Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total weighted average asset 32.00% 33.00%
Real Estate [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total weighted average asset 7.00% 6.00%