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Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Sep. 30, 2023
Energy Prepayment Obligations and Discounts on Sales    
Accounts Receivable, Allowance for Credit Loss, Current $ 1 $ 1 [1]
[1] Allowance for uncollectible accounts was less than $1 million at both September 30, 2024 and 2023, and therefore is not represented in the table above.