FAIR VALUE MEASUREMENTS - Summary of Nuclear Trust Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2025	Dec. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 4,858.9	$ 4,395.1
Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,589.0	2,297.8
Gross Unrealized Losses	(7.3)	(15.5)
Other-Than-Temporary Impairments	(22.6)	(26.0)
I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	4,858.9	4,395.1
I&M \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,589.0	2,297.8
Gross Unrealized Losses	(7.3)	(15.5)
Other-Than-Temporary Impairments	(22.6)	(26.0)
Cash and Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	25.0	23.3
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	0.0
Gross Unrealized Losses	0.0	0.0
Other-Than-Temporary Impairments	0.0	0.0
Cash and Cash Equivalents \| I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	25.0	23.3
Cash and Cash Equivalents \| I&M \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Other-Than-Temporary Impairments	0.0	0.0
United States Government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,385.1	1,322.8
United States Government \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	23.0	8.2
Gross Unrealized Losses	(0.3)	(5.3)
Other-Than-Temporary Impairments	(15.5)	(20.2)
United States Government \| I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,385.1	1,322.8
United States Government \| I&M \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	23.0	8.2
Gross Unrealized Losses	(0.3)	(5.3)
Other-Than-Temporary Impairments	(15.5)	(20.2)
Corporate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	352.9	211.3
Corporate Debt \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	6.0	0.7
Gross Unrealized Losses	(6.8)	(9.8)
Other-Than-Temporary Impairments	(7.1)	(5.8)
Corporate Debt \| I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	352.9	211.3
Corporate Debt \| I&M \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	6.0	0.7
Gross Unrealized Losses	(6.8)	(9.8)
Other-Than-Temporary Impairments	(7.1)	(5.8)
Subtotal Fixed Income Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,738.0	1,534.1
Subtotal Fixed Income Securities \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	29.0	8.9
Gross Unrealized Losses	(7.1)	(15.1)
Other-Than-Temporary Impairments	(22.6)	(26.0)
Subtotal Fixed Income Securities \| I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,738.0	1,534.1
Subtotal Fixed Income Securities \| I&M \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	29.0	8.9
Gross Unrealized Losses	(7.1)	(15.1)
Other-Than-Temporary Impairments	(22.6)	(26.0)
Equity Securities - Domestic
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,095.9	2,837.7
Equity Securities - Domestic \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,560.0	2,288.9
Gross Unrealized Losses	(0.2)	(0.4)
Other-Than-Temporary Impairments	0.0	0.0
Equity Securities - Domestic \| I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,095.9	2,837.7
Equity Securities - Domestic \| I&M \| Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,560.0	2,288.9
Gross Unrealized Losses	(0.2)	(0.4)
Other-Than-Temporary Impairments	$ 0.0	$ 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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