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Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - USD ($)
$ in Millions
Jun. 30, 2016
Dec. 31, 2015
Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives $ 6.5 $ 3.2
Liability Derivatives 9.4 3.7
Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 282.8 255.6
Liability Derivatives 538.5 143.0
Derivatives designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 248.9 257.2
Liability Derivatives 467.4 142.6
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 6.5 3.2
Liability Derivatives 9.4 3.7
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 4.1 3.2
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 2.4 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | 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 9.4 3.7
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 242.4 254.0
Liability Derivatives 458.0 138.9
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 238.8 253.8
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 3.6 0.2
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 456.3 137.5
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 1.7 1.4
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 40.4 1.6
Liability Derivatives 80.5 4.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 38.5 1.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 1.9 0.0
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 73.1 3.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 7.4 $ 1.0