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Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Sep. 30, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Allowance for uncollectible accounts $ 1 $ 1
Financing Receivable, after Allowance for Credit Loss 97 99
Accounts and Financing Receivable, Allowance for Credit Loss 2 2
Reported Value Measurement    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Financing Receivable, after Allowance for Credit Loss $ 104 $ 105