FAIR VALUE MEASUREMENTS - Narrative (Details) - Spent Nuclear Fuel and Decommissioning Trusts - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Adjusted cost of debt securities	$ 1,800	$ 1,700
Adjusted cost of equity securities	550	540
INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Adjusted cost of debt securities	1,800	1,700
Adjusted cost of equity securities	$ 550	$ 540