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DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details)
$ in Millions
Sep. 30, 2025
USD ($)
Instruments
Dec. 31, 2024
USD ($)
Instruments
Derivative assets    
Current derivative assets $ 37.5 $ 37.0
Long-term derivative assets 0.7 4.1
Total derivative assets $ 38.2 $ 41.1
Current derivative assets balance sheet location Other Other
Long-term derivative assets balance sheet location Other Assets, Noncurrent Other Assets, Noncurrent
Derivative liabilities    
Current derivative liabilities $ 26.4 $ 13.9
Long-term derivative liabilities 0.9 0.0
Total derivative liabilities $ 27.3 $ 13.9
Current derivative liabilities balance sheet location Other Liabilities, Current Other Liabilities, Current
Long-term derivative liabilities balance sheet location Other noncurrent liabilities Other noncurrent liabilities
Natural gas contracts    
Derivative assets    
Current derivative assets $ 26.8 $ 29.2
Derivative liabilities    
Current derivative liabilities 26.3 13.9
FTRs and TCRs    
Derivative assets    
Current derivative assets 10.7 7.8
Derivative liabilities    
Current derivative liabilities $ 0.1 $ 0.0
Designated as hedging instrument    
Derivative instruments    
Number of derivative instruments | Instruments 0 0