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Employee Benefit Plans (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2024	Oct. 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
Discontinued operations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1			$ 4	$ 5	$ 12
Non-service cost (credit)	(0)			(11)	13	(34)
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	2			3	8	10
Decrease in the accumulated postretirement benefit obligation, transferred upon closing	(1)				(1)
Defined benefit plan, loss on sale	1			1	5	4
Other Postretirement Benefits Plan | Closure of East Ohio Transaction
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Increase in the fair value of the plan assets due to remeasurement		$ 24	19
Increase in the fair value of the plan assets, transferred upon closing		5	36
Decrease in the accumulated postretirement benefit obligation		14	38
Decrease in the accumulated postretirement benefit obligation, transferred upon closing		$ 6	22
Increase (decrease) in net periodic benefit credit			1
Defined benefit plan, loss on sale			$ (9)
Other Postretirement Benefits Plan | Questar Gas and Wexpro
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Remeasurement, discount rate		5.74%
Defined benefit plan, loss on sale		$ 1
Other Postretirement Benefits Plan | Forecast | Questar Gas and Wexpro
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Increase (decrease) in net periodic benefit credit							$ 3
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]
Contributions to qualified defined benefit pension plans	33				40
Service cost	19			$ 24	63	$ 72
Decrease in the accumulated postretirement benefit obligation, transferred upon closing					4
Defined benefit plan, loss on sale	$ (3)				$ (16)
Pension Benefits | Subsequent Event
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Additional employer contributions								$ 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
Increase in the fair value of the plan assets due to remeasurement			555
Increase in the fair value of the plan assets, transferred upon closing			531
Increase (decrease) in net periodic benefit credit			$ 11
Remeasurement, discount rate			5.62%
Defined benefit plan, loss on sale			$ 147
Pension Benefits | Questar Gas and Wexpro
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Decrease in the pension benefit obligation due to remeasurement		251
Decrease in the pension benefit obligation, amount transferred upon closing		136
Increase in the fair value of the plan assets due to remeasurement		248
Increase in the fair value of the plan assets, transferred upon closing		$ 138
Remeasurement, discount rate		5.75%
Defined benefit plan, loss on sale		$ 49
Pension Benefits | Forecast | Questar Gas and Wexpro
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Increase (decrease) in net periodic benefit credit							$ 8

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Amount of increase (decrease) in obligation for pension benefits. Includes, but is not limited to, defined benefit or defined contribution plans. Excludes other postretirement benefits.

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