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Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
 6 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Fair Value [Member]
Level 2 [Member]
Dec. 31, 2012
Fair Value [Member]
Level 2 [Member]
Jun. 30, 2013
Carrying Value [Member]
Dec. 31, 2012
Carrying Value [Member]
Jun. 30, 2013
Fair Value, Measurements, Recurring [Member]
Jun. 30, 2013
Fair Value, Measurements, Recurring [Member]
Level 1 [Member]
Jun. 30, 2013
Fair Value, Measurements, Recurring [Member]
Level 2 [Member]
Jun. 30, 2013
Fair Value, Measurements, Recurring [Member]
Level 3 [Member]
Jun. 30, 2013
Fair Value, Measurements, Recurring [Member]
Fair Value [Member]
Level 3 [Member]
Mar. 31, 2013
Fair Value, Measurements, Recurring [Member]
Fair Value [Member]
Level 3 [Member]
Jun. 30, 2012
Fair Value, Measurements, Recurring [Member]
Fair Value [Member]
Level 3 [Member]
Mar. 31, 2012
Fair Value, Measurements, Recurring [Member]
Fair Value [Member]
Level 3 [Member]
Jun. 30, 2013
Fair Value, Measurements, Recurring [Member]
Fair Value [Member]
Level 3 [Member]
Jun. 30, 2012
Fair Value, Measurements, Recurring [Member]
Fair Value [Member]
Level 3 [Member]
Financial assets [Abstract]                                
Interest rate derivatives             $ 35.5 $ 0 $ 35.5 $ 0            
Commodity derivatives             60.8 20.5 40.2 0.1            
Financial assets             96.3 20.5 75.7 0.1            
Financial liabilities [Abstract]                                
Interest rate derivatives             12.4 0 12.4 0            
Commodity derivatives             46.9 8.9 37.7 0.3            
Financial liabilities             59.3 8.9 50.1 0.3            
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]                                
Balance, beginning of period                     (0.6) (1.5) 0.9 0.4 (1.5) 0.4
Total gains (losses) included in:                                
Net income                     (0.2) [1] (0.6) [1] (1.3) [1] 0.5 [1]    
Other comprehensive income (loss)                     0 0 6.0 0.5    
Settlements                     0.6 1.5 (0.7) (0.5)    
Balance, end of period                     (0.2) [2] (0.6) 4.9 0.9 (0.2) [2] 4.9
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities 1.2 21.6                 0.4   (2.0)   1.3 (1.9)
Financial Liabilities: [Abstract]                                
Fixed Rate Debt Principal Amount Fair Value Disclosure     $ 17,980 $ 18,420 $ 16,880 $ 16,180                    
[1] 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. There were $2.0 million and $1.9 million of unrealized losses included in these amounts for the three and six months ended June 30, 2012, respectively.
[2] There were no transfers into or out of Level 3 during the three or six months ended June 30, 2013.