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Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Financing Receivable, Past Due $ 125   $ 125    
Defined Benefit Plan, Other Cost (Credit) 65 $ 65 194 $ 193  
Prior Period Reclassification Adjustment     13    
Accounts Payable and Other Accrued Liabilities 12   12    
Other Operating Income (Expense), Net (8)        
Deferred Costs 6   6    
Fuel cost tax equivalents (8)        
Financing Receivable, after Allowance for Credit Loss 111   111   $ 123
Other Deferred Costs, Net 29   29    
Other Depreciation and Amortization 15        
Other Operating Activities, Cash Flow Statement (20)        
Carrying Value          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Financing Receivable, after Allowance for Credit Loss $ 125   $ 125   $ 138