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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($)
$ in Millions
Jun. 30, 2023
Dec. 31, 2022
Assets    
Derivative assets $ 8.7 $ 38.8
Liabilities    
Derivative liabilities 64.6 30.0
Fair value measurements on a recurring basis    
Assets    
Derivative assets 8.7 38.8
Liabilities    
Derivative liabilities 64.6  
Fair value measurements on a recurring basis | Level 1    
Assets    
Derivative assets 0.5 1.6
Liabilities    
Derivative liabilities 35.5  
Fair value measurements on a recurring basis | Level 2    
Assets    
Derivative assets 1.3 35.2
Liabilities    
Derivative liabilities 29.1  
Fair value measurements on a recurring basis | Level 3    
Assets    
Derivative assets 6.9 2.0
Liabilities    
Derivative liabilities 0.0  
Fair value measurements on a recurring basis | Natural gas contracts    
Assets    
Derivative assets 1.6 4.1
Liabilities    
Derivative liabilities 37.1 30.0
Fair value measurements on a recurring basis | Natural gas contracts | Level 1    
Assets    
Derivative assets 0.5 1.6
Liabilities    
Derivative liabilities 35.5 29.3
Fair value measurements on a recurring basis | Natural gas contracts | Level 2    
Assets    
Derivative assets 1.1 2.5
Liabilities    
Derivative liabilities 1.6 0.7
Fair value measurements on a recurring basis | Natural gas contracts | Level 3    
Assets    
Derivative assets 0.0 0.0
Liabilities    
Derivative liabilities 0.0 0.0
Fair value measurements on a recurring basis | FTRs    
Assets    
Derivative assets 6.9 2.0
Fair value measurements on a recurring basis | FTRs | Level 1    
Assets    
Derivative assets 0.0 0.0
Fair value measurements on a recurring basis | FTRs | Level 2    
Assets    
Derivative assets 0.0 0.0
Fair value measurements on a recurring basis | FTRs | Level 3    
Assets    
Derivative assets 6.9 2.0
Fair value measurements on a recurring basis | Coal contracts    
Assets    
Derivative assets 0.2 32.7
Liabilities    
Derivative liabilities 27.5  
Fair value measurements on a recurring basis | Coal contracts | Level 1    
Assets    
Derivative assets 0.0 0.0
Liabilities    
Derivative liabilities 0.0  
Fair value measurements on a recurring basis | Coal contracts | Level 2    
Assets    
Derivative assets 0.2 32.7
Liabilities    
Derivative liabilities 27.5  
Fair value measurements on a recurring basis | Coal contracts | Level 3    
Assets    
Derivative assets 0.0 $ 0.0
Liabilities    
Derivative liabilities $ 0.0