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Summary of Significant Accounting Policies - General (Details) People in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) People | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2015 USD ($)
Accounting Policies [Abstract] |  |  |  |  |
Cash and Cash Equivalents, at Carrying Value | $ 500 | $ 299 |  |  |
Allowance for uncollectible accounts - loans |  | 1 |  |  |
Revenue Capitalized During Pre-Commercial Operations | 0 | 0 | $ 11 |  |
Fuel Cost Capitalized During Pre-Commercial Operations |  | 19 |  |  |
Recorded cost for emission allowances granted by the Environmental Protection Agency | 0 |  |  |  |
Reimbursements from DOE | 89 |  |  |  |
Restricted Cash and Investments, Noncurrent | 21 | 23 |  |  |
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents | 521 | 322 | $ 322 | $ 311 |
Accounts receivable from DOE | 6 |  |  |  |
Appropriation-investment power program | $ 258 | 258 |  |  | $ 1,000
Population of Service Area [Line Items] |  |  |  |  |
Population of service area | People | 10 |  |  |  |
Total |  |  |  |  |
Accounting Policies [Abstract] |  |  |  |  |
Discounts reducing electricity sales | $ 4 | 46 |  |  |
MLGW |  |  |  |  |
Accounting Policies [Abstract] |  |  |  |  |
Deferred revenue, revenue recognized |  | $ 100 |  |  |

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