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