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Employee Benefit Plans (Narrative) (Detail) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 30, 2024 | Mar. 31, 2024 | Mar. 31, 2023 | Dec. 31, 2024
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined contribution plan operating expenses within discontinued operations |  | $ 23 |  |
Defined Contribution Plan Operating Expenses Within Discontinued Operations After Tax |  | 17 |  |
Defined contribution plan other operations and maintenance expense |  | 13 |  |
Defined contribution plan other operations and maintenance expense after tax |  | 10 |  |
Subsequent Event |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Contributions to qualified defined benefit pension plans | $ 7 |  |  |
Discontinued operations |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service cost |  | 3 | $ 4 |
Non-service cost (credit) |  | 14 | (11) |
Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service cost |  | 3 | 3 |
Defined benefit plan, loss on sale |  | 2 | 1 |
Other Postretirement Benefits Plan | Closure of East Ohio Transaction |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Decrease in the fair value of the plan assets due to remeasurement |  | 19 |  |
Decrease in the fair value of the plan assets, transferred upon closing |  | 36 |  |
Decrease in the accumulated postretirement benefit obligation |  | 38 |  |
Decrease in the accumulated postretirement benefit obligation, transferred upon closing |  | 22 |  |
Defined benefit plan, loss on sale |  | $ (9) |  |
Other Postretirement Benefits Plan | Forecast | Closure of East Ohio Transaction |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Increase (decrease) in net periodic benefit credit |  |  |  | $ 1
Other Postretirement Benefits Plan | Minimum | Closure of East Ohio Transaction |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Remeasurement, discount rate |  | 5.61% |  |
Other Postretirement Benefits Plan | Maximum | Closure of East Ohio Transaction |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Remeasurement, discount rate |  | 5.62% |  |
Pension Benefits |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Expected contributions to qualified defined benefit pension plans in next fiscal year |  | $ 46 |  |
Service cost |  | 22 | $ 24 |
Defined benefit plan, loss on sale |  | (6) |  |
Pension Benefits | Closure of East Ohio Transaction |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Decrease in the pension benefit obligation due to remeasurement |  | 419 |  |
Decrease in the pension benefit obligation, amount transferred upon closing |  | 195 |  |
Decrease in the fair value of the plan assets due to remeasurement |  | 555 |  |
Decrease in the fair value of the plan assets, transferred upon closing |  | $ 531 |  |
Remeasurement, discount rate |  | 5.62% |  |
Defined benefit plan, loss on sale |  | $ 147 |  |
Pension Benefits | Forecast | Closure of East Ohio Transaction |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Increase (decrease) in net periodic benefit credit |  |  |  | $ 11

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