FAIR VALUE MEASUREMENTS - Changes in Fair Value (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	$ 259	$ 166
Realized Gain (Loss) Included in Net Income (or Changes in Net Assets)	175	92
Unrealized Gain (Loss) Included in Net Income (or Changes in Net Assets) Relating to Assets Still Held at the Reporting Date	(4)	19
Realized and Unrealized Gains (Losses) Included in Other Comprehensive Income	8	5
Settlements	(319)	(168)
Transfers into Level 3	2	1
Transfers out of Level 3	1	1
Changes in Fair Value Allocated to Regulated Jurisdiction	10	7
Ending Balance	132	123
APPALACHIAN POWER COMPANY
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	81	35
Realized Gain (Loss) Included in Net Income (or Changes in Net Assets)	109	40
Unrealized Gain (Loss) Included in Net Income (or Changes in Net Assets) Relating to Assets Still Held at the Reporting Date	0	0
Realized and Unrealized Gains (Losses) Included in Other Comprehensive Income	0	0
Settlements	(172)	(66)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Changes in Fair Value Allocated to Regulated Jurisdiction	3	(1)
Ending Balance	21	8
INDIANA MICHIGAN POWER COMPANY
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	9	6
Realized Gain (Loss) Included in Net Income (or Changes in Net Assets)	7	9
Unrealized Gain (Loss) Included in Net Income (or Changes in Net Assets) Relating to Assets Still Held at the Reporting Date	0	0
Realized and Unrealized Gains (Losses) Included in Other Comprehensive Income	0	0
Settlements	(13)	(12)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Changes in Fair Value Allocated to Regulated Jurisdiction	(1)	1
Ending Balance	2	4
OHIO POWER COMPANY
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	(33)	(47)
Realized Gain (Loss) Included in Net Income (or Changes in Net Assets)	1	0
Unrealized Gain (Loss) Included in Net Income (or Changes in Net Assets) Relating to Assets Still Held at the Reporting Date	0	0
Realized and Unrealized Gains (Losses) Included in Other Comprehensive Income	0	0
Settlements	1	2
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Changes in Fair Value Allocated to Regulated Jurisdiction	(1)	(6)
Ending Balance	(32)	(51)
PUBLIC SERVICE COMPANY OF OKLAHOMA
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	41	20
Realized Gain (Loss) Included in Net Income (or Changes in Net Assets)	36	17
Unrealized Gain (Loss) Included in Net Income (or Changes in Net Assets) Relating to Assets Still Held at the Reporting Date	0	0
Realized and Unrealized Gains (Losses) Included in Other Comprehensive Income	0	0
Settlements	(61)	(28)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Changes in Fair Value Allocated to Regulated Jurisdiction	5	6
Ending Balance	21	15
SOUTHWESTERN ELECTRIC POWER COMPANY
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	35	17
Realized Gain (Loss) Included in Net Income (or Changes in Net Assets)	7	21
Unrealized Gain (Loss) Included in Net Income (or Changes in Net Assets) Relating to Assets Still Held at the Reporting Date	0	0
Realized and Unrealized Gains (Losses) Included in Other Comprehensive Income	0	0
Settlements	(29)	(31)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Changes in Fair Value Allocated to Regulated Jurisdiction	2	6
Ending Balance	$ 15	$ 13