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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Reclassificatons (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Reclassifications      
Restricted Cash   $ 12 $ 1
Defined Benefit Plan, Other Cost (Credit) $ 258 256 758
Reclass of Other, net   (22) (60)
Reclass of Other regulatory amortization and deferral   2 143
Other regulatory amortization and deferrals [Member]      
Reclassifications      
Deferred Fuel Cost   (30) 98
Reclass of fuel cost tax equivalents   (7) 5
Reclass of Other, net   39 40
Other, net [Member]      
Reclassifications      
Reclass of accounts payable   (9) (10)
Regulatory assets costs   (13) (50)
Payments to U S Treasury   (5) (5)
Reclass of short-term debt issues (redemptions)   (29) $ (9)
Other Long-Term Assets      
Reclassifications      
Restricted Cash   $ 13