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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Allowance for Uncollectible Accounts |  |  |
Revenue Capitalized During Pre-Commercial Operations | $ 0 | $ 11 | $ (22)
Period of time for customers to fulfill payment arrangements | 90 days |  |
Allowance for uncollectible accounts - receivables |  | 1 |
Loans receivable | $ 120 | 123 |
Allowance for uncollectible accounts - loans |  | 1 |
Fuel Cost Capitalized During Pre-Commercial Operations |  | $ 19 |

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