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Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - Commodity derivatives [Member] - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Derivatives, Fair Value [Line Items]    
Asset Derivatives $ 138.4 $ 161.9
Liability Derivatives 108.4 162.4
Derivatives designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 112.8 144.1
Liability Derivatives 93.9 126.1
Derivatives designated as hedging instruments [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 102.5 138.5
Derivatives designated as hedging instruments [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 10.3 5.6
Derivatives designated as hedging instruments [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 81.7 115.0
Derivatives designated as hedging instruments [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 12.2 11.1
Derivatives not designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 25.6 17.8
Liability Derivatives 14.5 36.3
Derivatives not designated as hedging instruments [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 23.6 15.9
Derivatives not designated as hedging instruments [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 2.0 1.9
Derivatives not designated as hedging instruments [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 13.2 33.2
Derivatives not designated as hedging instruments [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 1.3 $ 3.1