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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

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