v3.26.3
GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Finite-Lived Intangible Liabilities          
Net carrying amount $ 535.2   $ 535.2   $ 580.3
Amortization 15.1 $ 14.7 $ 45.1 $ 43.4  
Period of amortization     5 years    
Income Statement Location [Axis]: us-gaap:OtherCostAndExpenseOperating          
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    
Income Statement Location [Axis]: us-gaap: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 59.3   59.3    
2030 59.1   59.1    
WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 763.1   763.1   763.1
Intangible liabilities, accumulated amortization (227.9)   (227.9)   (182.8)
Net carrying amount 535.2   535.2   580.3
PPAs | WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 751.2   751.2   751.2
Intangible liabilities, accumulated amortization (220.9)   (220.9)   (176.5)
Net carrying amount 530.3   $ 530.3   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     9 years    
Proxy revenue swap | WECI          
Finite-Lived Intangible Liabilities          
Gross carrying amount 7.2   $ 7.2   7.2
Intangible liabilities, accumulated amortization (5.5)   (5.5)   (4.9)
Net carrying amount $ 1.7   $ 1.7   2.3
Proxy revenue swap | Upstream          
Finite-Lived Intangible Liabilities          
Weighted average remaining useful life     2 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 (1.5)   (1.5)   (1.4)
Net carrying amount $ 3.2   $ 3.2   $ 3.3
Interconnection agreements | Tatanka Ridge and Bishop Hill III          
Finite-Lived Intangible Liabilities          
Weighted average remaining useful life     14 years