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Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Dec. 31, 2013
Total gains (losses) included in:      
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities $ 7.8 $ (12.3)  
Fair Value, Measurements, Recurring [Member]
      
Financial assets [Abstract]      
Interest rate derivatives 26.1   32.6
Commodity derivatives 39.8   41.3
Financial assets 65.9   73.9
Financial liabilities [Abstract]      
Interest rate derivatives 5.2   7.8
Commodity derivatives 38.3   55.1
Financial liabilities 43.5   62.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
      
Financial assets [Abstract]      
Interest rate derivatives 0   0
Commodity derivatives 14.2   17.2
Financial assets 14.2   17.2
Financial liabilities [Abstract]      
Interest rate derivatives 0   0
Commodity derivatives 20.2   30.8
Financial liabilities 20.2   30.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
      
Financial assets [Abstract]      
Interest rate derivatives 26.1   32.6
Commodity derivatives 17.0   20.2
Financial assets 43.1   52.8
Financial liabilities [Abstract]      
Interest rate derivatives 5.2   7.8
Commodity derivatives 17.2   23.6
Financial liabilities 22.4   31.4
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
      
Financial assets [Abstract]      
Interest rate derivatives 0   0
Commodity derivatives 8.6   3.9
Financial assets 8.6   3.9
Financial liabilities [Abstract]      
Interest rate derivatives 0   0
Commodity derivatives 0.9   0.7
Financial liabilities 0.9   0.7
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]      
Financial asset (liability) balance, net, beginning of period 3.2 (1.5)  
Total gains (losses) included in:      
Net income 4.6 [1] (0.6) [1]  
Settlements (0.1) 1.5  
Financial asset (liability) balance, net, end of period 7.7 (0.6)  
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities $ 4.5 $ 0.9  
[1] There were unrealized gains of $4.5 million and $0.9 million included in these amounts for the three months ended March 31, 2014 and 2013, respectively.