PROPERTY, PLANT AND EQUIPMENT - Proportionate Share of Operating Costs (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Utility Plant in Service	$ 1,926	$ 1,921
Construction Work in Progress	3	5
Accumulated Depreciation	589	539
SOUTHWESTERN ELECTRIC POWER COMPANY
Property, Plant and Equipment [Line Items]
Utility Plant in Service	3,019	3,015
Construction Work in Progress	7	6
Accumulated Depreciation	$ 717	$ 637
Flint Creek Generating Station (Unit No. 1) | Coal
Property, Plant and Equipment [Line Items]
Percent of Ownership	50.00%	50.00%
Utility Plant in Service	$ 406	$ 404
Construction Work in Progress	2	4
Accumulated Depreciation	$ 208	$ 189
Flint Creek Generating Station (Unit No. 1) | Coal | SOUTHWESTERN ELECTRIC POWER COMPANY
Property, Plant and Equipment [Line Items]
Percent of Ownership	50.00%	50.00%
Utility Plant in Service	$ 406	$ 404
Construction Work in Progress	2	4
Accumulated Depreciation	$ 208	$ 189
Turk Generating Plant | Coal
Property, Plant and Equipment [Line Items]
Percent of Ownership	73.30%	73.30%
Utility Plant in Service	$ 1,520	$ 1,517
Construction Work in Progress	1	1
Accumulated Depreciation	$ 381	$ 350
Turk Generating Plant | Coal | SOUTHWESTERN ELECTRIC POWER COMPANY
Property, Plant and Equipment [Line Items]
Percent of Ownership	73.30%	73.30%
Utility Plant in Service	$ 1,520	$ 1,517
Construction Work in Progress	1	1
Accumulated Depreciation	$ 381	$ 350
Rockport Generating Plant | AEGCo
Property, Plant and Equipment [Line Items]
Percent of Ownership	50.00%
Rockport Generating Plant | Coal | INDIANA MICHIGAN POWER COMPANY
Property, Plant and Equipment [Line Items]
Percent of Ownership	50.00%	50.00%
Utility Plant in Service	$ 1,353	$ 1,345
Construction Work in Progress	16	11
Accumulated Depreciation	$ 1,341	$ 1,182
North Central Wind Energy Facilities | PUBLIC SERVICE COMPANY OF OKLAHOMA
Property, Plant and Equipment [Line Items]
Percent of Ownership	45.50%
North Central Wind Energy Facilities | SOUTHWESTERN ELECTRIC POWER COMPANY
Property, Plant and Equipment [Line Items]
Percent of Ownership	54.50%
North Central Wind Energy Facilities | Wind | PUBLIC SERVICE COMPANY OF OKLAHOMA
Property, Plant and Equipment [Line Items]
Percent of Ownership	45.50%	45.50%
Utility Plant in Service	$ 912	$ 912
Construction Work in Progress	3	1
Accumulated Depreciation	$ 101	$ 78
North Central Wind Energy Facilities | Wind | SOUTHWESTERN ELECTRIC POWER COMPANY
Property, Plant and Equipment [Line Items]
Percent of Ownership	54.50%	54.50%
Utility Plant in Service	$ 1,093	$ 1,094
Construction Work in Progress	4	1
Accumulated Depreciation	$ 128	$ 98