v3.26.1
GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Finite-Lived Intangible Liabilities          
Net carrying amount $ 550.3   $ 550.3   $ 580.3
Amortization 15.0 $ 14.8 $ 30.0 $ 28.7  
Period of amortization     5 years    
Amortization to be recorded as an increase to operating revenues          
Amortization to be recorded in the next five years          
2026 59.9   $ 59.9    
2027 59.9   59.9    
2028 59.9   59.9    
2029 59.9   59.9    
2030 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          
2026 0.2   0.2    
2027 0.2   0.2    
2028 0.2   0.2    
2029 0.2   0.2    
2030 0.2   0.2    
WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 763.1   763.1   763.1
Intangible liabilities, accumulated amortization (212.8)   (212.8)   (182.8)
Net carrying amount 550.3   550.3   580.3
PPAs | WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 751.2   751.2   751.2
Intangible liabilities, accumulated amortization (206.1)   (206.1)   (176.5)
Net carrying amount 545.1   $ 545.1   574.7
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
Intangible liabilities, accumulated amortization (5.3)   (5.3)   (4.9)
Net carrying amount $ 1.9   $ 1.9   2.3
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
Intangible liabilities, accumulated amortization (1.4)   (1.4)   (1.4)
Net carrying amount $ 3.3   $ 3.3   $ 3.3
Interconnection agreements | Tatanka Ridge and Bishop Hill III          
Finite-Lived Intangible Liabilities          
Weighted average remaining useful life     14 years