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Proprietary Capital (Tables) | 12 Months Ended
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Sep. 30, 2020
Equity [Abstract] |
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] | Accumulated Other Comprehensive Income (Loss) The items included in AOCI consist of market valuation adjustments for certain derivative instruments. See Note 15 — Risk Management Activities and Derivative Transactions.
Stockholders' Equity Note [Abstract] |
Summary of Proprietary Capital Activity | The table below summarizes TVA's activities related to appropriated funds and retained earnings.
Summary of Proprietary Capital Activity At or for the years ended September 30
| 2020 |  | 2019
| Power Program |  | Nonpower Programs |  | Power Program |  | Nonpower Programs
Appropriation Investment | $ | 258 |  |  | $ | 4,351 |  |  | $ | 258 |  |  | $ | 4,351 |
Proprietary Capital |  |  |  |  |  |  |
Balance at beginning of year | 10,823 |  |  | (3,795) |  |  | 9,404 |  |  | (3,787) |
Net income (loss) for year | 1,360 |  |  | (8) |  |  | 1,425 |  |  | (8) |
Return on power program appropriation investment | (6) |  |  | — |  |  | (6) |  |  | — |
Balance at end of year | 12,177 |  |  | (3,803) |  |  | 10,823 |  |  | (3,795) |
Net proprietary capital at September 30 | $ | 12,435 |  |  | $ | 548 |  |  | $ | 11,081 |  |  | $ | 556 |

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