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Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 118.6  
Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 172.6 $ 161.9
Liability Derivatives 150.5 162.4
Derivatives designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 154.9 144.1
Liability Derivatives 169.7 126.1
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 0.0
Liability Derivatives 23.7 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 11.8 0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 11.9 0.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 154.9 144.1
Liability Derivatives 146.0 126.1
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 149.3 138.5
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 5.6 5.6
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 139.2 115.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 6.8 11.1
Derivatives not designated as hedging instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 17.7 17.8
Liability Derivatives 99.4 36.3
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 0.0
Liability Derivatives 94.9 0.0
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 0.0
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 0.0 0.0
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 47.2 0.0
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 47.7 0.0
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 17.7 17.8
Liability Derivatives 4.5 36.3
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 16.7 15.9
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives 1.0 1.9
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives 4.2 33.2
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 0.3 $ 3.1