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Equity Investments - Investment in Nobles 2 (Details)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
USD ($)
MW
Jun. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
MW
Jun. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Investment in Nobles 2 [Line Items]          
Equity Investment Balance $ 317.8   $ 317.8   $ 301.2
Cash Investments     17.1 $ 66.6  
Equity Earnings 5.1 $ 6.4 9.9 11.6  
Net Loss Attributable to Non-Controlling Interest $ (6.2) $ (3.2) $ (14.6) $ (5.0)  
Nobles 2 [Member]          
Investment in Nobles 2 [Line Items]          
Ownership Percentage 49.00%   49.00%    
Equity Investment Balance $ 166.2   $ 166.2   152.2
Cash Investments     17.1    
Distributed Earnings     (2.5)    
Equity Earnings [1]     (0.6)    
Net Loss Attributable to Non-Controlling Interest     $ 3.4    
ATC [Member]          
Investment in Nobles 2 [Line Items]          
Ownership Percentage 8.00%   8.00%    
Equity Investment Balance $ 151.6   $ 151.6   $ 149.0
Distributed Earnings     (8.5)    
Equity Earnings     $ 10.5    
Wind Turbine Generators [Member] | Tenaska [Member] | Minnesota Power [Member] | Tenaska PPA [Member]          
Investment in Nobles 2 [Line Items]          
Generating Capacity Counterparty Owned (MW) | MW 250   250    
Long-term Contract for Purchase of Electric Power, Term of Contract (Years)     20 years    
[1] The Company also recorded net loss attributable to non-controlling interest of $3.4 million related to its investment in Nobles 2.