FAIR VALUE MEASUREMENTS - Summary of Nuclear Trust Investments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2026	Dec. 31, 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 4,803	$ 4,916
Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,514	2,649
Gross Unrealized Losses	(14)	(9)
Other-Than-Temporary Impairments	(22)	(21)
INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	4,803	4,916
INDIANA MICHIGAN POWER COMPANY \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,514	2,649
Gross Unrealized Losses	(14)	(9)
Other-Than-Temporary Impairments	(22)	(21)
Cash and Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	27	29
Cash and Cash Equivalents \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Other-Than-Temporary Impairments	0	0
Cash and Cash Equivalents \| INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	27	29
Cash and Cash Equivalents \| INDIANA MICHIGAN POWER COMPANY \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Other-Than-Temporary Impairments	0	0
United States Government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,351	1,351
United States Government \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	13	22
Gross Unrealized Losses	(2)	(1)
Other-Than-Temporary Impairments	(16)	(15)
United States Government \| INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,351	1,351
United States Government \| INDIANA MICHIGAN POWER COMPANY \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	13	22
Gross Unrealized Losses	(2)	(1)
Other-Than-Temporary Impairments	(16)	(15)
Corporate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	379	376
Corporate Debt \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	3	6
Gross Unrealized Losses	(10)	(7)
Other-Than-Temporary Impairments	(6)	(6)
Corporate Debt \| INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	379	376
Corporate Debt \| INDIANA MICHIGAN POWER COMPANY \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	3	6
Gross Unrealized Losses	(10)	(7)
Other-Than-Temporary Impairments	(6)	(6)
Subtotal Fixed Income Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,730	1,727
Subtotal Fixed Income Securities \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	16	28
Gross Unrealized Losses	(12)	(8)
Other-Than-Temporary Impairments	(22)	(21)
Subtotal Fixed Income Securities \| INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,730	1,727
Subtotal Fixed Income Securities \| INDIANA MICHIGAN POWER COMPANY \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	16	28
Gross Unrealized Losses	(12)	(8)
Other-Than-Temporary Impairments	(22)	(21)
Equity Securities - Domestic
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,046	3,160
Equity Securities - Domestic \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,498	2,621
Gross Unrealized Losses	(2)	(1)
Other-Than-Temporary Impairments	0	0
Equity Securities - Domestic \| INDIANA MICHIGAN POWER COMPANY
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,046	3,160
Equity Securities - Domestic \| INDIANA MICHIGAN POWER COMPANY \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,498	2,621
Gross Unrealized Losses	(2)	(1)
Other-Than-Temporary Impairments	$ 0	$ 0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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