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Risk Management Activities and Derivative Transactions Offsetting of Derivative Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Sep. 30, 2017
Offsetting Assets [Line Items]    
Gross Amounts Offset in the Balance Sheet [1] $ 0  
Net Amounts of Assets Presented in the Balance Sheet [2] 17  
Total derivatives not subject to master netting or similar arrangement [2] 16  
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement 17  
Letter of Credit    
Offsetting Assets [Line Items]    
Amount of letters of credit outstanding for credit facilities 1,000 $ 1,200
Currency Swap    
Offsetting Assets [Line Items]    
Gross Amounts of Recognized Assets [3] 1  
Gross Amounts Offset in the Balance Sheet [1],[3] 0  
Net Amounts of Assets Presented in the Balance Sheet [2],[3] $ 1  
Commodity derivatives under FTP    
Offsetting Assets [Line Items]    
Gross Amounts of Recognized Assets   10
Gross Amounts Offset in the Balance Sheet [1]   0
Net Amounts of Assets Presented in the Balance Sheet [2]   $ 10
[1] Amounts primarily include counterparty netting of derivative contracts, margin account deposits for futures commission merchants transactions, and cash collateral received or paid in accordance with the accounting guidance for derivatives and hedging transactions.
[2] There are no derivative contracts subject to a master netting arrangement or similar agreement that are not offset in the consolidated balance sheets.
[3] Letters of credit of approximately $1.0 billion and $1.2 billion were posted as collateral at December 31, 2017, and September 30, 2017, respectively, to partially secure the liability positions of one of the currency swaps and one of the interest rate swaps in accordance with the collateral requirements for these derivatives.