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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Assets (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Sep. 30, 2018
Offsetting Assets [Line Items]    
Commodity derivatives not subject to master netting or similar arrangement [1]   $ 72
Letter of Credit    
Offsetting Assets [Line Items]    
Amount of letters of credit outstanding $ 1,300 $ 900
Other Contract    
Offsetting Assets [Line Items]    
Gross Amounts of Recognized Assets, subject to master netting or similar arrangements 12  
Net Amounts of Assets Presented in the Balance Sheet [1] $ 12  
[1] There are no derivative contracts subject to a master netting arrangement or similar agreement that are not offset on the Consolidated Balance Sheets.