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Pension and Other Benefit Plans - Changes in Accumulated Postretirement Benefit Obligation, Plan Assets and Funded Status (Detail) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Change in Plan Assets        
Benefits paid $ (257)      
Postretirement medical plan        
Change in Projected Benefit Obligations        
Projected benefit obligation at beginning of year   $ 1,106 $ 1,213  
Service cost   29 32 $ 29
Interest cost   45 43 46
Contribution by plan participants   8 8  
Actuarial (gains) losses   65 (128)  
Benefits paid   (60) (62)  
Projected benefit obligation at end of year 1,193 1,193 1,106 1,213
Change in Plan Assets        
Plan assets at fair value at beginning of year   997 1,094  
Actual return on plan assets   240 (44)  
Company contributions     1  
Contributions by plan participants   8 8  
Benefits paid   (60) (62)  
Plan assets at fair value at end of year 1,185 1,185 997 $ 1,094
Unfunded Liability (8) (8) (109)  
Amounts Recognized in Accumulated Other Comprehensive Loss        
Actuarial losses (98) (98) 14  
Prior service credit (158) (158) (186)  
Total amount recognized in accumulated other comprehensive income $ (256) $ (256) $ (172)