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Variable Interest Entity: (Details)
$ in Millions
 12 Months Ended
Apr. 14, 2016
Dec. 31, 2023
USD ($)
MW
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Variable Interest Entity [Line Items]        
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage 49.90%      
Net income attributable to non-controlling interest   $ (13.8) $ (12.4) $ (14.5)
Number of Days Following the End of the Quarter When Distributions of Net Income Attributable to this Non-controlling Interest are Due   30 days    
Current assets   $ 826.6 1,071.7  
Property, plant and equipment   7,119.3 6,798.0  
Current liabilities   1,185.1 1,666.7  
Variable Interest Entity, Primary Beneficiary        
Variable Interest Entity [Line Items]        
Current assets   15.1 12.8  
Property, plant and equipment   166.8 178.8  
Current liabilities   4.8 5.4  
Power Generation        
Variable Interest Entity [Line Items]        
Net income attributable to non-controlling interest   $ (13.8) $ (12.4) $ (14.5)
Pueblo Airport Generation | Subsidiary of Common Parent        
Variable Interest Entity [Line Items]        
Electric Generation Capacity, Megawatts | MW   200