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Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Total gains (losses) included in:      
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities $ 20.3 $ (20.1)  
Fair Value, Measurements, Recurring [Member]      
Financial assets [Abstract]      
Interest rate derivatives 39.3   $ 36.5
Commodity derivatives:      
Value before application of CME Rule 814 233.4    
Impact of CME Rule 814 change (192.2)    
Total commodity derivatives 41.2   541.4
Total 80.5   577.9
Financial liabilities [Abstract]      
Liquidity option agreement 275.1   269.6
Interest rate derivatives 2.0   1.1
Commodity derivatives:      
Value before application of CME Rule 814 209.1    
Impact of CME Rule 814 change (164.6)    
Total commodity derivatives 44.5   739.4
Total 321.6   1,010.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]      
Financial assets [Abstract]      
Interest rate derivatives 0.0   0.0
Commodity derivatives:      
Value before application of CME Rule 814 57.6    
Impact of CME Rule 814 change (57.6)    
Total commodity derivatives 0.0   84.5
Total 0.0   84.5
Financial liabilities [Abstract]      
Liquidity option agreement 0.0   0.0
Interest rate derivatives 0.0   0.0
Commodity derivatives:      
Value before application of CME Rule 814 35.4    
Impact of CME Rule 814 change (35.4)    
Total commodity derivatives 0.0   136.8
Total 0.0   136.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]      
Financial assets [Abstract]      
Interest rate derivatives 39.3   36.5
Commodity derivatives:      
Value before application of CME Rule 814 174.3    
Impact of CME Rule 814 change (134.6)    
Total commodity derivatives 39.7   455.2
Total 79.0   491.7
Financial liabilities [Abstract]      
Liquidity option agreement 0.0   0.0
Interest rate derivatives 2.0   1.1
Commodity derivatives:      
Value before application of CME Rule 814 172.9    
Impact of CME Rule 814 change (129.2)    
Total commodity derivatives 43.7   602.3
Total 45.7   603.4
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]      
Financial assets [Abstract]      
Interest rate derivatives 0.0   0.0
Commodity derivatives:      
Value before application of CME Rule 814 1.5    
Impact of CME Rule 814 change 0.0    
Total commodity derivatives 1.5   1.7
Total 1.5   1.7
Financial liabilities [Abstract]      
Liquidity option agreement 275.1   269.6
Interest rate derivatives 0.0   0.0
Commodity derivatives:      
Value before application of CME Rule 814 0.8    
Impact of CME Rule 814 change 0.0    
Total commodity derivatives 0.8   0.3
Total 275.9   $ 269.9
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]      
Financial liability balance, net, beginning of period (268.2) (246.7)  
Total gains (losses) included in:      
Transfers out of Level 3 0.0 0.1  
Financial liability balance, net, end of period (274.4) (242.3)  
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities (0.6) 0.6  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]      
Total gains (losses) included in:      
Net income [1] 0.7 0.7  
Settlements (1.4) (0.1)  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]      
Total gains (losses) included in:      
Net income (5.5) 2.2  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]      
Total gains (losses) included in:      
Other comprehensive income $ 0.0 $ 1.5  
[1] There were unrealized losses of $0.6 million and unrealized gains of $0.6 million included in these amounts for the three months ended March 31, 2017 and 2016, respectively.