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Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Financing Receivable, after Allowance for Credit Loss $ 107 $ 93
Allowance for uncollectible accounts 1  
Loans and Leases Receivable, Allowance 5 1
Carrying Value    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Financing Receivable, after Allowance for Credit Loss $ 117 $ 105