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Proprietary Capital Proprietary Capital (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Amount of appropriation investment that was repaid | $ 1,000 |  |  |  |  |  |  |  | $ 1,000 |  |
Remaining appropriation investment | 258 |  |  |  |  |  |  |  | 258 |  |
Balance at beginning of year |  |  |  | $ 9,133 |  |  |  | $ 8,420 | 9,133 | $ 8,420 | $ 7,203
Net income (loss) | (101) | $ 470 | $ 462 | 288 | $ 139 | $ 233 | $ 211 | 102 | 1,119 | 685 | 1,233
Return on power program appropriation investment |  |  |  |  |  |  |  |  | (5) | (5) | (6)
Balance at end of year | 10,283 |  |  |  | 9,133 |  |  |  | 10,283 | 9,133 | $ 8,420
Net proprietary capital at September 30 | $ 10,283 |  |  |  | $ 9,133 |  |  |  | $ 10,283 | $ 9,133 |
Computed average interest rate payable | 2.09% |  |  |  | 2.00% |  |  |  | 2.09% | 2.00% | 2.04%
Nonpower Programs Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | 4,351 |  |  |  |  | $ 4,351 |  |
Balance at end of year | $ 4,351 |  |  |  | $ 4,351 |  |  |  | 4,351 | $ 4,351 |
Power Program Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | 258 |  |  |  | 258 | 258 | 258 | $ 258
Net income (loss) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 | 0
Balance at end of year | 258 |  |  |  | 258 |  |  |  | 258 | 258 | 258
Power Program Retained Earnings |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | 8,282 |  |  |  | 7,594 | 8,282 | 7,594 | 6,357
Net income (loss) |  |  |  |  |  |  |  |  | 1,127 | 693 | 1,243
Return on power program appropriation investment |  |  |  |  |  |  |  |  | (5) | (5) | (6)
Balance at end of year | 9,404 |  |  |  | 8,282 |  |  |  | 9,404 | 8,282 | 7,594
Net proprietary capital at September 30 | 9,662 |  |  |  | 8,540 |  |  |  | 9,662 | 8,540 |
Nonpower Programs Appropriation Investment, Net |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | 572 |  |  |  | 580 | 572 | 580 | 590
Net income (loss) |  |  |  |  |  |  |  |  | (8) | (8) | (10)
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 | 0
Balance at end of year | 564 |  |  |  | 572 |  |  |  | 564 | 572 | 580
Nonpower Programs Retained Earnings |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | $ (3,779) |  |  |  | $ (3,771) | (3,779) | (3,771) |
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 |
Balance at end of year | (3,787) |  |  |  | (3,779) |  |  |  | (3,787) | (3,779) | (3,771)
Net proprietary capital at September 30 | $ 564 |  |  |  | $ 572 |  |  |  | 564 | 572 |
Affiliated Entity |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Return on power program appropriation investment |  |  |  |  |  |  |  |  | $ (5) | $ (5) | $ (6)

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