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Employee Benefits - Summary of Changes in Projected Benefit Obligation, Plan Assets, Funded Status, Amounts Recognized in Consolidated Balance Sheet and Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Plans [Member]      
Change In Projected Benefit Obligation      
Benefit obligation at beginning of measurement period $ 5,720 $ 5,073  
Service cost 208 187 $ 177
Interest cost 224 220 211
Actuarial loss (gain) (440) 430  
Lump sum settlements (50) (45)  
Benefit payments (155) (145)  
Benefit obligation at end of measurement period 5,507 5,720 5,073
Change In Fair Value Of Plan Assets      
Balance at beginning of period 5,482 3,769  
Actual return on plan assets (365) 665  
Employer contributions 24 1,238  
Lump sum settlements (50) (45)  
Benefit payments (155) (145)  
Balance at end of period 4,936 5,482 3,769
Funded (Unfunded) Status (571) (238)  
Noncurrent benefit asset   270  
Current benefit liability (23) (23)  
Noncurrent benefit liability (548) (485)  
Recognized amount (571) (238)  
Net actuarial gain (loss) (1,981) (1,822)  
Recognized amount (1,981) (1,822)  
Postretirement Welfare Plan [Member]      
Change In Projected Benefit Obligation      
Benefit obligation at beginning of measurement period 68 75  
Interest cost 2 2 3
Participants' contributions 8 8  
Actuarial loss (gain) (7) (1)  
Benefit payments (18) (18)  
Federal subsidy on benefits paid 1 2  
Benefit obligation at end of measurement period 54 68 75
Change In Fair Value Of Plan Assets      
Balance at beginning of period 87 82  
Actual return on plan assets   10  
Employer contributions 5 5  
Participants' contributions 7 8  
Benefit payments (18) (18)  
Balance at end of period 81 87 $ 82
Funded (Unfunded) Status 27 19  
Noncurrent benefit asset 26 19  
Recognized amount 26 19  
Net actuarial gain (loss) 66 68  
Net prior service credit (cost) 18 22  
Recognized amount $ 84 $ 90