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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Assets    
Derivative assets $ 38.2 $ 41.1
Liabilities    
Derivative liabilities 27.3 13.9
Fair value measurements on a recurring basis    
Assets    
Derivative assets 38.2 41.1
Investments held in rabbi trust 41.0 52.1
Liabilities    
Derivative liabilities 27.3  
Fair value measurements on a recurring basis | Level 1    
Assets    
Derivative assets 6.1 19.6
Investments held in rabbi trust 41.0 52.1
Liabilities    
Derivative liabilities 17.1  
Fair value measurements on a recurring basis | Level 2    
Assets    
Derivative assets 21.4 13.7
Investments held in rabbi trust 0.0 0.0
Liabilities    
Derivative liabilities 10.1  
Fair value measurements on a recurring basis | Level 3    
Assets    
Derivative assets 10.7 7.8
Investments held in rabbi trust 0.0 0.0
Liabilities    
Derivative liabilities 0.1  
Fair value measurements on a recurring basis | Natural gas contracts    
Assets    
Derivative assets 27.5 33.3
Liabilities    
Derivative liabilities 27.2 13.9
Fair value measurements on a recurring basis | Natural gas contracts | Level 1    
Assets    
Derivative assets 6.1 19.6
Liabilities    
Derivative liabilities 17.1 7.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2    
Assets    
Derivative assets 21.4 13.7
Liabilities    
Derivative liabilities 10.1 6.8
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 and TCRs    
Assets    
Derivative assets 10.7 7.8
Liabilities    
Derivative liabilities 0.1  
Fair value measurements on a recurring basis | FTRs and TCRs | Level 1    
Assets    
Derivative assets 0.0 0.0
Liabilities    
Derivative liabilities 0.0  
Fair value measurements on a recurring basis | FTRs and TCRs | Level 2    
Assets    
Derivative assets 0.0 0.0
Liabilities    
Derivative liabilities 0.0  
Fair value measurements on a recurring basis | FTRs and TCRs | Level 3    
Assets    
Derivative assets 10.7 $ 7.8
Liabilities    
Derivative liabilities $ 0.1