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Risk Management Activities and Derivative Transactions Offsetting of Derivative Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Sep. 30, 2019
Offsetting Assets [Line Items]    
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement $ 1,763 $ 2,025
Gross amounts of recognized liabilities 1,697 1,972
Total derivatives not subject to master netting or similar arrangement 4 12
Forward Contract Derivative Asset, at Fair Value 2 22
Derivative Liability, Not Subject to Master Netting Arrangement 66 53
Letter of Credit    
Offsetting Assets [Line Items]    
Amount of letters of credit outstanding for credit facilities 1,200 1,300
Currency Swap    
Offsetting Assets [Line Items]    
Gross amounts of recognized liabilities 132 208
Interest Rate Contract    
Offsetting Assets [Line Items]    
Gross amounts of recognized liabilities 1,565 1,764
Fair Value, Inputs, Level 2    
Offsetting Assets [Line Items]    
Forward Contract Derivative Asset, at Fair Value $ 2 $ 22