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Employee Benefit Plans - Assumptions Used to Determine Projected and Accumulated Benefit Obligations of Domestic Defined Benefit Pension and Postretirement Benefit Plans (Detail)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pre-65      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate 7.00% 7.00%  
Ultimate cost trend rate 4.50% 4.50%  
Year that the rate reaches the ultimate cost trend rate: 2024 2023 2023
Post-65      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate 7.80% 7.80%  
Ultimate cost trend rate 4.50% 4.50%  
Year that the rate reaches the ultimate cost trend rate: 2024 2024 2024
Postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.40% 3.90%  
Deferred compensation and excess benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.40% 3.90%  
Annual increase in Medicare Part B premium 6.00% 6.00%  
Projected and accumulated benefit obligations | Pre-65      
Defined Benefit Plan Disclosure [Line Items]      
Year that the rate reaches the ultimate cost trend rate: 2024 2024  
Projected and accumulated benefit obligations | Post-65      
Defined Benefit Plan Disclosure [Line Items]      
Year that the rate reaches the ultimate cost trend rate: 2024 2024