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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Cash, Cash Equivalents and Restricted Cash:        
Cash and cash equivalents $ 5.1 $ 63.1    
Restricted cash 65.2 354.5    
Total cash, cash equivalents and restricted cash shown in the Statements of Consolidated Cash Flows 70.3 417.6 $ 34.9 $ 74.4
Scenario, Previously Reported [Member]        
Cash, Cash Equivalents and Restricted Cash:        
Decrease in restricted cash   (338.6) (15.9)  
Allowance for Doubtful Accounts [Member]        
Movement in valuation allowances and reserves [Roll Forward]        
Balance at beginning of period 11.3 12.1 13.9  
Charged to costs and expenses 2.7 2.3 0.8  
Deductions (1.9) (3.1) (2.6)  
Balance at end of period $ 12.1 $ 11.3 $ 12.1  
Minor Investment [Member] | Minimum [Member]        
Consolidation Policy [Abstract]        
Equity method of ownership interest 3.00%      
Minor Investment [Member] | Maximum [Member]        
Consolidation Policy [Abstract]        
Equity method of ownership interest 50.00%      
Major Investment [Member] | Minimum [Member]        
Consolidation Policy [Abstract]        
Equity method of ownership interest 20.00%      
Major Investment [Member] | Maximum [Member]        
Consolidation Policy [Abstract]        
Equity method of ownership interest 50.00%      
Initial Margin Requirement [Member]        
Cash, Cash Equivalents and Restricted Cash:        
Restricted cash $ 33.4      
Variation Margin Requirement [Member]        
Cash, Cash Equivalents and Restricted Cash:        
Restricted cash $ 31.8