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DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details)
$ in Millions
Mar. 31, 2023
USD ($)
Instruments
Dec. 31, 2022
USD ($)
Instruments
Derivative assets    
Current derivative assets $ 7.0 $ 59.2
Long-term derivative assets 0.7 15.6
Total derivative assets $ 7.7 $ 74.8
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 $ 156.5 $ 88.2
Long-term derivative liabilities 13.3 8.4
Total derivative liabilities $ 169.8 $ 96.6
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 $ 2.7 $ 32.5
Long-term derivative assets 0.3 0.0
Derivative liabilities    
Current derivative liabilities 147.3 88.2
Long-term derivative liabilities 7.8 8.4
FTRs    
Derivative assets    
Current derivative assets 3.0 7.8
Derivative liabilities    
Current derivative liabilities 0.0 0.0
Coal contracts    
Derivative assets    
Current derivative assets 1.3 18.9
Long-term derivative assets 0.4 15.6
Derivative liabilities    
Current derivative liabilities 9.2 0.0
Long-term derivative liabilities $ 5.5 $ 0.0
Designated as hedging instrument    
Derivative instruments    
Number of derivative instruments | Instruments 0 0