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FAIR VALUE MEASUREMENTS - Securities Activity (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Purchases of Investments $ 602.7 $ 590.0
I&M    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Purchases of Investments 601.2 588.5
Spent Nuclear Fuel and Decommissioning Trusts    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Proceeds from Investment Sales 576.5 569.5
Purchases of Investments 601.2 588.5
Gross Realized Gains on Investment Sales 1.9 5.4
Gross Realized Losses on Investment Sales 0.7 1.2
Spent Nuclear Fuel and Decommissioning Trusts | I&M    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Proceeds from Investment Sales 576.5 569.5
Purchases of Investments 601.2 588.5
Gross Realized Gains on Investment Sales 1.9 5.4
Gross Realized Losses on Investment Sales $ 0.7 $ 1.2