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Employee Benefit Plans (Net Periodic Benefit (Credit) Cost and Amounts Recognized in Other Comprehensive Income and Regulatory Assets and Liabilities) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Pension Benefits      
Service cost $ 170 $ 173 $ 162
Interest cost 317 351 394
Expected return on plan assets (834) (777) (708)
Amortization of prior service (credit) cost   1 1
Amortization of net actuarial loss 193 206 172
Settlements, curtailments and special termination benefits 10 14 72
Net periodic benefit (credit) cost (144) (32) 93
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities:      
Current year net actuarial (gain) loss (782) 166 16
Settlements and curtailments (36) (81) 6
Amortization of net actuarial loss (193) (206) (172)
Amortization of prior service credit (cost)   (1) (1)
Total recognized in other comprehensive income and regulatory assets and liabilities $ (1,011) $ (122) $ (151)
Significant assumptions used to determine periodic cost:      
Weighted average rate of increase for compensation 4.53% 4.23% 4.20%
Pension Benefits | Minimum      
Significant assumptions used to determine periodic cost:      
Discount rate 2.73% 2.77% 3.57%
Expected long-term rate of return on plan assets 7.00% 7.00% 7.00%
Crediting interest rate for cash balance and similar plans 1.93% 2.31% 2.77%
Pension Benefits | Maximum      
Significant assumptions used to determine periodic cost:      
Discount rate 3.29% 3.63% 4.43%
Expected long-term rate of return on plan assets 8.45% 8.60% 8.65%
Crediting interest rate for cash balance and similar plans 2.15% 2.83% 3.63%
Other Postretirement Benefits      
Service cost $ 25 $ 28 $ 26
Interest cost 46 58 68
Expected return on plan assets (173) (156) (140)
Amortization of prior service (credit) cost (42) (49) (52)
Amortization of net actuarial loss 4 6 10
Settlements, curtailments and special termination benefits   (59) 42
Net periodic benefit (credit) cost (140) (172) (46)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities:      
Current year net actuarial (gain) loss (282) (110) (98)
Prior service (credit) cost (13) (6) 2
Settlements and curtailments   59  
Amortization of net actuarial loss (4) (6) (10)
Amortization of prior service credit (cost) 42 49 52
Total recognized in other comprehensive income and regulatory assets and liabilities $ (257) $ (14) $ (54)
Significant assumptions used to determine periodic cost:      
Expected long-term rate of return on plan assets 8.45% 8.50% 8.50%
Health care cost trend rate [1] 6.25% 6.25%  
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) [1] 5.00% 5.00% 5.00%
Other Postretirement Benefits | Minimum      
Significant assumptions used to determine periodic cost:      
Discount rate 2.69% 3.07% 4.05%
Health care cost trend rate [1]     6.50%
Year that the rate reaches the ultimate trend rate [1] 2026 2025 2023
Other Postretirement Benefits | Maximum      
Significant assumptions used to determine periodic cost:      
Discount rate 2.80% 3.52% 4.41%
Health care cost trend rate [1]     6.60%
Year that the rate reaches the ultimate trend rate [1] 2027 2026 2025
[1] Assumptions used to determine net periodic cost for the following year.