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Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Sep. 30, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Allowance for uncollectible accounts $ 1 $ 1
Financing Receivable, after Allowance for Credit Loss 97 96
Loans and Leases Receivable, Allowance 3 3
Reported Value Measurement    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Financing Receivable, after Allowance for Credit Loss $ 105 $ 104