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Derivatives (Details-BS) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Derivatives, Fair Value [Line Items]      
Derivative Liability, Fair Value, Gross Asset   $ 0  
Derivative Liability, Fair Value, Gross Liability   140  
Derivative Liability, Fair Value, Net   (140)  
Derivative Asset (Liability), Fair Value, Gross Asset $ 9    
Derivative Liability (Asset), Fair Value, Gross Liability 237    
Derivative, Fair Value, Net (228)    
Commodity Contract [Member] | Other Long-Term Assets [Member] | Not Designated as Hedging Instrument [Member]      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Fair Value, Gross Asset 9    
Derivative Asset, Fair Value, Gross Liability 0    
Derivative Asset, Fair Value, Net 9    
Commodity Contract [Member] | Other Current Liabilities [Member] | Not Designated as Hedging Instrument [Member]      
Derivatives, Fair Value [Line Items]      
Derivative Liability, Fair Value, Gross Asset 0 0  
Derivative Liability, Fair Value, Gross Liability 231 138  
Derivative Liability, Fair Value, Net (231) (138)  
Commodity Contract [Member] | Deferred Credits and Other Liabilities [Member] | Not Designated as Hedging Instrument [Member]      
Derivatives, Fair Value [Line Items]      
Derivative Liability, Fair Value, Gross Asset 0 0  
Derivative Liability, Fair Value, Gross Liability 6 2  
Derivative Liability, Fair Value, Net $ (6) $ (2)  
Unsecured Debt [Member]      
Derivatives, Fair Value [Line Items]      
Debt Instrument, Face Amount     $ 1,000