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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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Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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Amount of contribution expected to be received by defined benefit plan from employer in remainder of current fiscal year. Excludes contribution previously paid by employer in current fiscal year.

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Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit.

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