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Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2019
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Prior Period Reclassification Adjustment $ 65 $ 13 $ 128  
Accounts Payable and Other Accrued Liabilities 12 12    
Other Operating Income (Expense), Net (5)      
Deferred Costs 39 39    
Fuel cost tax equivalents (6)      
Financing Receivable, Net 136 136   $ 123
Allowance for Notes, Loans and Financing Receivable, Noncurrent 1 1    
Other Deferred Costs, Net 19 19    
Other Depreciation and Amortization 26      
Other Operating Activities, Cash Flow Statement (17)      
Carrying Value        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Financing Receivable, Net $ 151 $ 151   $ 138