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Summary of Significant Accounting Policies - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019
Portion at Other than Fair Value Measurement [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Financing Receivable, after Allowance for Credit Loss | $ 105 | $ 131
Financing Receivable, after Allowance for Credit Loss | $ 93 | 120
Period of time for customers to fulfill payment arrangements | 90 days |
Allowance for uncollectible accounts - receivables |  | 1
Allowance for uncollectible accounts - loans |  | 1
Fuel Cost Capitalized During Pre-Commercial Operations |  | $ 19

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