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FAIR VALUE MEASUREMENTS - Securities Activity (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Purchases of Investments |  |  | $ 2,407.0 | $ 2,398.0
I&M |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Purchases of Investments |  |  | 2,386.5 | 2,389.0
Spent Nuclear Fuel and Decommissioning Trusts |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Proceeds from Investment Sales | $ 1,037.8 | $ 1,181.9 | 2,332.2 | 2,336.0
Purchases of Investments | 1,056.5 | 1,201.9 | 2,386.5 | 2,389.0
Gross Realized Gains on Investment Sales | 112.5 | 108.4 | 116.8 | 118.7
Gross Realized Losses on Investment Sales | 1.5 | 0.7 | 4.0 | 6.1
Spent Nuclear Fuel and Decommissioning Trusts | I&M |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Proceeds from Investment Sales | 1,037.8 | 1,181.9 | 2,332.2 | 2,336.0
Purchases of Investments | 1,056.5 | 1,201.9 | 2,386.5 | 2,389.0
Gross Realized Gains on Investment Sales | 112.5 | 108.4 | 116.8 | 118.7
Gross Realized Losses on Investment Sales | $ 1.5 | $ 0.7 | $ 4.0 | $ 6.1

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