v3.26.3
FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Assets    
Derivative assets $ 34.2 $ 26.3
Liabilities    
Derivative liabilities 57.2 32.5
Fair value measurements on a recurring basis    
Assets    
Derivative assets 34.2 26.3
Investments held in rabbi trust 44.2 42.0
Liabilities    
Derivative liabilities   32.5
Fair value measurements on a recurring basis | Level 1    
Assets    
Derivative assets 2.3 1.5
Investments held in rabbi trust 44.2 42.0
Liabilities    
Derivative liabilities   23.3
Fair value measurements on a recurring basis | Level 2    
Assets    
Derivative assets 17.2 18.3
Investments held in rabbi trust 0.0 0.0
Liabilities    
Derivative liabilities   8.4
Fair value measurements on a recurring basis | Level 3    
Assets    
Derivative assets 14.7 6.5
Investments held in rabbi trust 0.0 0.0
Liabilities    
Derivative liabilities   0.8
Fair value measurements on a recurring basis | Natural gas contracts    
Assets    
Derivative assets 19.5 19.8
Liabilities    
Derivative liabilities 57.2 31.7
Fair value measurements on a recurring basis | Natural gas contracts | Level 1    
Assets    
Derivative assets 2.3 1.5
Liabilities    
Derivative liabilities 48.2 23.3
Fair value measurements on a recurring basis | Natural gas contracts | Level 2    
Assets    
Derivative assets 17.2 18.3
Liabilities    
Derivative liabilities 9.0 8.4
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 14.7 6.5
Liabilities    
Derivative liabilities   0.8
Fair value measurements on a recurring basis | FTRs | Level 1    
Assets    
Derivative assets 0.0 0.0
Liabilities    
Derivative liabilities   0.0
Fair value measurements on a recurring basis | FTRs | Level 2    
Assets    
Derivative assets 0.0 0.0
Liabilities    
Derivative liabilities   0.0
Fair value measurements on a recurring basis | FTRs | Level 3    
Assets    
Derivative assets $ 14.7 6.5
Liabilities    
Derivative liabilities   $ 0.8