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Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 13.5  
Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 127.8 $ 161.9
Liability Derivatives 116.2 162.4
Derivatives designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 116.5 144.1
Liability Derivatives 120.6 126.1
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 0.0
Liability Derivatives 13.5 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 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] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 6.7 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 6.8 0.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 116.5 144.1
Liability Derivatives 107.1 126.1
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 116.5 138.5
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 5.6
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 107.1 115.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 0.0 11.1
Derivatives not designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 11.3 17.8
Liability Derivatives 9.1 36.3
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 11.3 17.8
Liability Derivatives 9.1 36.3
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 10.7 15.9
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.6 1.9
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 8.6 33.2
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 0.5 $ 3.1