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Risk Management Activities and Derivative Transactions Offsetting of Derivative Assets (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Sep. 30, 2017
Offsetting Assets [Line Items]    
Total derivatives not subject to master netting or similar arrangement [1] $ 27  
Forward Contract Derivative Asset, at Fair Value 52 $ 19
Letter of Credit    
Offsetting Assets [Line Items]    
Amount of letters of credit outstanding for credit facilities 900 1,200
Commodity derivatives under FTP    
Offsetting Assets [Line Items]    
Gross Amounts of Recognized Assets   10
Gross Amounts Offset in the Balance Sheet [2]   0
Net Amounts of Assets Presented in the Balance Sheet [1]   10
Fair Value, Inputs, Level 2    
Offsetting Assets [Line Items]    
Forward Contract Derivative Asset, at Fair Value $ 52 $ 19
[1] There are no derivative contracts subject to a master netting arrangement or similar agreement that are not offset in the consolidated balance sheets.
[2] 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.