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Pension and Other Benefit Plans - Changes in Accumulated Postretirement Benefit Obligation, Plan Assets and Funded Status (Detail) - Postretirement medical plan - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in Projected Benefit Obligations      
Projected benefit obligation at beginning of year $ 1,108 $ 1,103  
Service cost 29 30 $ 42
Interest cost 46 47 48
Contribution by plan participants 8 8  
Actuarial gains 71 (32)  
Benefits paid (49) (48)  
Projected benefit obligation at end of year 1,213 1,108 1,103
Change in Plan Assets      
Plan assets at fair value at beginning of year 952 884  
Actual return on plan assets 143 68  
Company contributions 40 40  
Contributions by plan participants 8 8  
Benefits paid (49) (48)  
Plan assets at fair value at end of year 1,094 952 $ 884
Unfunded Liability (119) (156)  
Amounts Recognized in Accumulated Other Comprehensive Loss      
Actuarial losses 36 62  
Prior service credit (215) (243)  
Total amount recognized in accumulated other comprehensive income $ (179) $ (181)