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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plans [Member]      
Change In Projected Benefit Obligation      
Benefit obligation at beginning of measurement period $ 5,507 $ 5,720  
Service cost 192 208 $ 187
Interest cost 249 224 220
Actuarial loss (gain) 1,100 (440)  
Lump sum settlements (56) (50)  
Benefit payments (163) (155)  
Benefit obligation at end of measurement period 6,829 5,507 5,720
Change In Fair Value Of Plan Assets      
Balance at beginning of period 4,936 5,482  
Actual return on plan assets 1,095 (365)  
Employer contributions 26 24  
Lump sum settlements (56) (50)  
Benefit payments (163) (155)  
Balance at end of period 5,838 4,936 5,482
Funded (Unfunded) Status (991) (571)  
Current benefit liability (25) (23)  
Noncurrent benefit liability (966) (548)  
Recognized amount (991) (571)  
Net actuarial gain (loss) (2,271) (1,981)  
Recognized amount (2,271) (1,981)  
Postretirement Welfare Plan [Member]      
Change In Projected Benefit Obligation      
Benefit obligation at beginning of measurement period 54 68  
Interest cost 2 2 2
Participants' contributions 7 8  
Actuarial loss (gain) (4) (7)  
Benefit payments (13) (18)  
Federal subsidy on benefits paid 1 1  
Benefit obligation at end of measurement period 47 54 68
Change In Fair Value Of Plan Assets      
Balance at beginning of period 81 87  
Actual return on plan assets 6    
Employer contributions 4 5  
Participants' contributions 6 7  
Benefit payments (13) (18)  
Balance at end of period 84 81 $ 87
Funded (Unfunded) Status 37 27  
Noncurrent benefit asset 37 27  
Recognized amount 37 27  
Net actuarial gain (loss) 68 66  
Net prior service credit (cost) 14 18  
Recognized amount $ 82 $ 84