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Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Energy Prepayment Obligations and Discounts on Sales      
Accounts Receivable, Allowance for Credit Loss, Current $ 1 $ 1  
Recognition of Deferred Revenue $ 0 $ 0 $ 10