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Pension and Other Postretirement Benefits (OPEB) - Schedule of Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost (included in O&M Expense) $ 22 $ 24 $ 66 $ 71
Interest Cost 57 57 173 169
Expected Return on Plan Assets (77) (80) (231) (241)
Amortization of Net Prior Service Costs 0 0 0 0
Amortization of Net Actuarial Loss (Gain) 16 17 47 53
Non-Service Components of Pension and OPEB Costs (Credits) (4) (6) (11) (19)
Total Net Periodic Benefit Costs (Credits) 18 18 55 52
OPEB [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost (included in O&M Expense) 0 1 1 2
Interest Cost 9 9 27 28
Expected Return on Plan Assets (8) (9) (25) (26)
Amortization of Net Prior Service Costs 1 0 2 1
Amortization of Net Actuarial Loss (Gain) (1) 0 (3) (1)
Non-Service Components of Pension and OPEB Costs (Credits) 1 0 1 2
Total Net Periodic Benefit Costs (Credits) $ 1 $ 1 $ 2 $ 4