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Employee Benefit Plans (Narrative) (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Jun. 30, 2024 | Mar. 31, 2024 | Sep. 30, 2025 | Sep. 30, 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 |  |
Expected contributions to qualified defined benefit pension plans for remainder of fiscal year | $ 19 |  |  |  | $ 19 |
East Ohio |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Remeasured pension and other postretirement benefit plans amount |  |  |  | 202 |  |
Remeasured pension and other postretirement benefit plans after tax amount |  |  |  | $ 151 |  |
Questar Gas |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Remeasured pension and other postretirement benefit plans amount |  |  | $ 15 |  |  |
Remeasured pension and other postretirement benefit plans after tax amount |  |  | $ 11 |  |  |
Discontinued Operations |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Service cost |  | $ 1 |  |  |  | $ 5
Non-service cost (credit) |  | 0 |  |  |  | 13
Other Postretirement Benefits Plan |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Service cost | 3 | 2 |  |  | 8 | 8
Other Postretirement Benefits Plan | Questar Gas |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Remeasurement, discount rate |  |  | 5.74% |  |  |
Other Postretirement Benefits Plan | Minimum | East Ohio |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Remeasurement, discount rate |  |  |  | 5.61% |  |
Other Postretirement Benefits Plan | Maximum | East Ohio |  |  |  |  |  |
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] |  |  |  |  |  |
Contributions to qualified defined benefit pension plans | 9 |  |  |  | 16 |
Service cost | $ 19 | $ 19 |  |  | $ 57 | $ 63
Pension Benefits | East Ohio |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Remeasurement, discount rate |  |  |  | 5.62% |  |
Pension Benefits | Questar Gas |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Remeasurement, discount rate |  |  | 5.75% |  |  |

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