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Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives   $ 12.4
Liability Derivatives   120.1
Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives $ 711.7 229.2
Liability Derivatives 789.9 246.3
Derivatives designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 645.8 223.3
Liability Derivatives 718.3 360.2
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 12.4
Liability Derivatives 0.0 120.1
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 12.4
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 0.0 109.1
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 0.0 11.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 645.8 210.9
Liability Derivatives 718.3 240.1
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 637.4 210.5
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 8.4 0.4
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 706.0 234.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 12.3 6.1
Derivatives not designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 65.9 18.3
Liability Derivatives 71.6 6.2
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 65.9 18.3
Liability Derivatives 71.6 6.2
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 57.5 18.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 8.4 0.2
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 64.1 6.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 7.5 $ 0.1