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Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2014
Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives $ 7.6  
Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 242.8 $ 226.6
Liability Derivatives 197.1 147.4
Derivatives designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 232.8 217.9
Liability Derivatives 186.7 145.3
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 7.6 0.0
Liability Derivatives 0.0 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 7.4 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.2 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 0.0 0.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 225.2 217.9
Liability Derivatives 186.7 145.3
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 216.9 217.9
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 8.3 0.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 178.5 145.3
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 8.2 0.0
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 17.6 8.7
Liability Derivatives 10.4 2.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 17.5 8.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.1 0.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 9.4 0.7
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 1.0 $ 1.4