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Summary of Significant Accounting Policies General and Basis of Presentation (Details) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2021 USD ($) People | Dec. 31, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($)
Accounting Policies [Abstract] |  |  |  |
Cash and Cash Equivalents, at Carrying Value | $ 507 |  | $ 499 |
Regulatory Assets | 7,956 |  | 8,152 |
Appropriation-investment power program |  |  | 1,000 |
Financing Receivable, after Allowance for Credit Loss | $ 99 |  | 94 |
Population of TVA's service area | People | 10 |  |  |
Allowance for uncollectible accounts | $ 1 |  |  |
Depreciation expense | 453 | $ 345 |  |
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents | 526 | 529 | 518 | $ 521
Cash and Cash Equivalents, Period Increase (Decrease) |  | 8 |  |
Restricted Cash and Cash Equivalents, Noncurrent | 19 |  | $ 19 |
Property, Plant and Equipment [Line Items] |  |  |  |
Depreciation expense | 453 | $ 345 |  |
Increase due to new depreciation rates |  |  |  |
Accounting Policies [Abstract] |  |  |  |
Depreciation expense | 98 |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Depreciation expense | $ 98 |  |  |

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