INCOME TAXES - Operating Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	$ 155	$ 110
PUBLIC SERVICE COMPANY OF OKLAHOMA
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	44	28
SOUTHWESTERN ELECTRIC POWER COMPANY
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	45	$ 36
Arkansas
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	15
Arkansas | SOUTHWESTERN ELECTRIC POWER COMPANY
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	15
Illinois
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	5
Kentucky
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	11
Louisiana
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	41
Louisiana | SOUTHWESTERN ELECTRIC POWER COMPANY
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	41
Michigan
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	1
Ohio Municipal
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	62
Oklahoma
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	54
Oklahoma | PUBLIC SERVICE COMPANY OF OKLAHOMA
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	55
Tennessee
Effective Income Tax Rate Reconciliation [Line Items]
Future income tax benefit	$ 4