FAIR VALUE MEASUREMENTS - Significant Unobservable Inputs (Details) - USD ($)	Mar. 31, 2026	Dec. 31, 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	$ 479,000,000	$ 617,000,000
Fair value, liabilities	295,000,000	310,000,000
APPALACHIAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	22,000,000	82,000,000
Fair value, liabilities	7,000,000	3,000,000
INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	3,000,000	10,000,000
Fair value, liabilities	0	0
OHIO POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	0	0
Fair value, liabilities	32,000,000	33,000,000
PUBLIC SERVICE COMPANY OF OKLAHOMA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	21,000,000	42,000,000
Fair value, liabilities	35,000,000	28,000,000
SOUTHWESTERN ELECTRIC POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	15,000,000	35,000,000
Fair value, liabilities	13,000,000	11,000,000
Energy Contracts | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	207,000,000	224,000,000
Fair value, liabilities	137,000,000	144,000,000
Energy Contracts | Level 3 | OHIO POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	0	0
Fair value, liabilities	$ 32,000,000	$ 33,000,000
Energy Contracts | Level 3 | Low | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	6.87	5.65
Input/Range, liability	6.87	5.65
Energy Contracts | Level 3 | Low | OHIO POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	21.59	21.44
Input/Range, liability	21.59	21.44
Energy Contracts | Level 3 | High | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	126.85	141.75
Input/Range, liability	126.85	141.75
Energy Contracts | Level 3 | High | OHIO POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	84.40	85.92
Input/Range, liability	84.40	85.92
Energy Contracts | Level 3 | Weighted Average | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	50.12	50.61
Input/Range, liability	50.12	50.61
Energy Contracts | Level 3 | Weighted Average | OHIO POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	50.10	50.10
Input/Range, liability	50.10	50.10
FTRs | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	$ 69,000,000	$ 187,000,000
Fair value, liabilities	7,000,000	8,000,000
FTRs | Level 3 | APPALACHIAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	22,000,000	81,000,000
Fair value, liabilities	1,000,000	0
FTRs | Level 3 | INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	3,000,000	9,000,000
Fair value, liabilities	1,000,000	0
FTRs | Level 3 | PUBLIC SERVICE COMPANY OF OKLAHOMA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	21,000,000	43,000,000
Fair value, liabilities	0	2,000,000
FTRs | Level 3 | SOUTHWESTERN ELECTRIC POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, assets	16,000,000	37,000,000
Fair value, liabilities	$ 1,000,000	$ 2,000,000
FTRs | Level 3 | Low | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(32.49)	(32.49)
Input/Range, liability	(32.49)	(32.49)
FTRs | Level 3 | Low | APPALACHIAN POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(0.39)	(0.26)
Input/Range, liability	(0.39)	(0.26)
FTRs | Level 3 | Low | INDIANA MICHIGAN POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(0.75)	(0.46)
Input/Range, liability	(0.75)	(0.46)
FTRs | Level 3 | Low | PUBLIC SERVICE COMPANY OF OKLAHOMA | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(32.49)	(32.49)
Input/Range, liability	(32.49)	(32.49)
FTRs | Level 3 | Low | SOUTHWESTERN ELECTRIC POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(32.49)	(32.49)
Input/Range, liability	(32.49)	(32.49)
FTRs | Level 3 | High | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	35.84	21.68
Input/Range, liability	35.84	21.68
FTRs | Level 3 | High | APPALACHIAN POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	11.22	17.55
Input/Range, liability	11.22	17.55
FTRs | Level 3 | High | INDIANA MICHIGAN POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	12.16	21.68
Input/Range, liability	12.16	21.68
FTRs | Level 3 | High | PUBLIC SERVICE COMPANY OF OKLAHOMA | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	7.80	8.54
Input/Range, liability	7.80	8.54
FTRs | Level 3 | High | SOUTHWESTERN ELECTRIC POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	7.80	8.54
Input/Range, liability	7.80	8.54
FTRs | Level 3 | Weighted Average | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(0.24)	0.49
Input/Range, liability	(0.24)	0.49
FTRs | Level 3 | Weighted Average | APPALACHIAN POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	2.27	3.47
Input/Range, liability	2.27	3.47
FTRs | Level 3 | Weighted Average | INDIANA MICHIGAN POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	1.02	1.60
Input/Range, liability	1.02	1.60
FTRs | Level 3 | Weighted Average | PUBLIC SERVICE COMPANY OF OKLAHOMA | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(7.34)	(5.49)
Input/Range, liability	(7.34)	(5.49)
FTRs | Level 3 | Weighted Average | SOUTHWESTERN ELECTRIC POWER COMPANY | Forward Market Price
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Input/Range, asset	(7.34)	(5.49)
Input/Range, liability	(7.34)	(5.49)