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Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - USD ($)
$ in Millions
Jun. 30, 2015
Dec. 31, 2014
Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives $ 5.7  
Liability Derivatives 5.5  
Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 132.3 $ 226.6
Liability Derivatives 131.3 147.4
Derivatives designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 123.8 217.9
Liability Derivatives 127.2 145.3
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 5.7 0.0
Liability Derivatives 5.5 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 5.7 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 0.0 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 5.5 0.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 118.1 217.9
Liability Derivatives 121.7 145.3
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 117.6 217.9
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.5 0.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 121.0 145.3
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 0.7 0.0
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 14.2 8.7
Liability Derivatives 9.6 2.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 13.8 8.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.4 0.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 8.2 0.7
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 1.4 $ 1.4