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Variable Interest Entities (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
MW
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Variable interest entities      
Equity investment $ 1,553.4 $ 1,443.9  
ATC      
Variable interest entities      
Equity method investment, ownership interest (as a percent) 60.00%    
Equity investment $ 1,515.8 1,443.9  
ATC distributions receivable 39.9 35.2  
Accounts payable for services received from ATC $ 31.2 28.7  
ATC HoldCo      
Variable interest entities      
Equity method investment, ownership interest (as a percent) 75.00%    
Equity investment $ 37.6    
Purchased power agreement      
Variable interest entities      
Firm capacity from purchased power agreement (in megawatts) | MW 236    
Minimum energy requirements over remaining term of purchased power agreement (in megawatts) | MW 0    
Remaining term of purchased power agreement (in years) 4 years    
Residual guarantee associated with purchased power agreement $ 0.0    
Required payments over remaining term of purchased power agreement 71.4    
Total capacity and lease payments under purchased power agreement $ 18.0 $ 54.2 $ 53.6