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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 6 Months Ended
Jun. 30, 2014
Mar. 31, 2014
Jun. 30, 2013
Mar. 31, 2013
Jun. 30, 2014
Jun. 30, 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         $ 6.2 $ 1.2  
Fair Value, Measurements, Recurring [Member]
              
Financial assets [Abstract]              
Interest rate derivatives 27.3       27.3   32.6
Commodity derivatives 44.1       44.1   41.3
Financial assets 71.4       71.4   73.9
Financial liabilities [Abstract]              
Interest rate derivatives 2.6       2.6   7.8
Commodity derivatives 66.2       66.2   55.1
Financial liabilities 68.8       68.8   62.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
              
Financial assets [Abstract]              
Interest rate derivatives 0       0   0
Commodity derivatives 18.0       18.0   17.2
Financial assets 18.0       18.0   17.2
Financial liabilities [Abstract]              
Interest rate derivatives 0       0   0
Commodity derivatives 32.1       32.1   30.8
Financial liabilities 32.1       32.1   30.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
              
Financial assets [Abstract]              
Interest rate derivatives 27.3       27.3   32.6
Commodity derivatives 22.0       22.0   20.2
Financial assets 49.3       49.3   52.8
Financial liabilities [Abstract]              
Interest rate derivatives 2.6       2.6   7.8
Commodity derivatives 32.6       32.6   23.6
Financial liabilities 35.2       35.2   31.4
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
              
Financial assets [Abstract]              
Interest rate derivatives 0       0   0
Commodity derivatives 4.1       4.1   3.9
Financial assets 4.1       4.1   3.9
Financial liabilities [Abstract]              
Interest rate derivatives 0       0   0
Commodity derivatives 1.5       1.5   0.7
Financial liabilities 1.5       1.5   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 7.7 3.2 (0.6) (1.5) 3.2 (1.5)  
Total gains (losses) included in:              
Net income (3.3) [1] 4.6 [1] (0.2) [1] (0.6) [1]      
Settlements (1.8) (0.1) 0.6 1.5      
Financial asset (liability) balance, net, end of period 2.6 [2] 7.7 (0.2) [2] (0.6) 2.6 [2] (0.2) [2]  
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities $ (5.0)   $ 0.4   $ (0.5) $ 1.3  
[1] There were $5.0 million and $0.5 million of unrealized losses included in these amounts for the three and six months ended June 30, 2014, respectively. There were unrealized gains of $0.4 million and $1.3 million included in these amounts for the three and six months ended June 30, 2013, respectively.
[2] There were no transfers into or out of Level 3 during the three or six months ended June 30, 2014 and 2013.