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GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Finite-Lived Intangible Liabilities          
Net carrying amount $ 595.0   $ 595.0   $ 566.8
Amortization 14.7 $ 13.4 $ 43.4 $ 40.2  
Period of amortization     5 years    
Amortization to be recorded as an increase to operating revenues          
Amortization to be recorded in the next five years          
2025 57.9   $ 57.9    
2026 59.9   59.9    
2027 59.9   59.9    
2028 59.9   59.9    
2029 59.9   59.9    
Amortization to be recorded as a decrease to other operation and maintenance          
Amortization to be recorded in the next five years          
2025 0.2   0.2    
2026 0.2   0.2    
2027 0.2   0.2    
2028 0.2   0.2    
2029 0.2   0.2    
WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 763.1   763.1   691.5
Accumulated amortization (168.1)   (168.1)   (124.7)
Net carrying amount 595.0   595.0   566.8
PPAs | WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 751.2   751.2   679.6
Accumulated amortization (162.0)   (162.0)   (119.3)
Net carrying amount 589.2   $ 589.2   560.3
PPAs | Blooming Grove, Tatanka Ridge, Jayhawk, Thunderhead, Samson I, Sapphire Sky, Delilah I, and Hardin III          
Finite-Lived Intangible Liabilities          
Weighted average remaining useful life     10 years    
Proxy revenue swap | WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 7.2   $ 7.2   7.2
Accumulated amortization (4.8)   (4.8)   (4.2)
Net carrying amount $ 2.4   $ 2.4   3.0
Proxy revenue swap | Upstream          
Finite-Lived Intangible Liabilities          
Weighted average remaining useful life     3 years    
Length of proxy revenue contract, in years 10 years   10 years    
Interconnection agreements | WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount $ 4.7   $ 4.7   4.7
Accumulated amortization (1.3)   (1.3)   (1.2)
Net carrying amount $ 3.4   $ 3.4   $ 3.5
Interconnection agreements | Tatanka Ridge and Bishop Hill III          
Finite-Lived Intangible Liabilities          
Weighted average remaining useful life     15 years