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Actuarial Assumptions Used to Determine Benefit Obligations (Detail)
Dec. 31, 2017
Dec. 31, 2016
Pension Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 3.65% 4.15%
Average salary increases   4.00%
Post-Retirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 3.79% 4.33%