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Pension Plans - Weighted-Average Asset Allocations of Plan Assets (Detail)
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of pension plan assets 100.00% 100.00%
Equity Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of pension plan assets 66.00% 61.00%
Debt Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of pension plan assets 32.00% 35.00%
Cash and Equivalents [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of pension plan assets 2.00% 4.00%