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Proprietary Capital Proprietary Capital (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2013
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Amount of appropriation investment that must be repaid | $ 1,000 |  |  |  |  |  |  |  | $ 1,000 |  |
Remaining appropriation investment | 258 |  |  |  |  |  |  |  | 258 |  |
Balance at beginning of year |  |  |  | $ 6,104 |  |  |  | $ 5,647 | 6,104 | $ 5,647 | $ 5,326
Net income (loss) | 502 | $ 32 | $ 496 | 81 | $ 322 | $ (81) | $ 295 | (67) | 1,111 | 469 | 271
Return of power program appropriation investment |  |  |  |  |  |  |  |  |  | 10 | 20
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 5 | 4 | 7
Balance at end of year | 7,203 |  |  |  | 6,104 |  |  |  | 7,203 | 6,104 | $ 5,647
Net proprietary capital at September 30 | $ 7,203 |  |  |  | $ 6,104 |  |  |  | $ 7,203 | $ 6,104 |
Computed average interest rate payable | 2.04% |  |  |  | 1.97% |  |  |  | 2.04% | 1.97% | 2.10%
Nonpower Programs Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | 4,351 |  |  |  | 4,351 | $ 4,351 | $ 4,351 |
Return of power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 |
Balance at end of year | $ 4,351 |  |  |  | $ 4,351 |  |  |  | 4,351 | 4,351 | $ 4,351
Power Program Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | 258 |  |  |  | 268 | 258 | 268 | 288
Net income (loss) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Return of power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 10 | 20
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 | 0
Balance at end of year | 258 |  |  |  | 258 |  |  |  | 258 | 258 | 268
Power Program Retained Earnings |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | 5,240 |  |  |  | 4,767 | 5,240 | 4,767 | 4,492
Net income (loss) |  |  |  |  |  |  |  |  | 1,122 | 477 | 282
Return of power program appropriation investment |  |  |  |  |  |  |  |  |  | 0 | 0
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 5 | 4 | 7
Balance at end of year | 6,357 |  |  |  | 5,240 |  |  |  | 6,357 | 5,240 | 4,767
Net proprietary capital at September 30 | 6,615 |  |  |  | 5,498 |  |  |  | 6,615 | 5,498 |
Nonpower Programs Retained Earnings |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | $ (3,750) |  |  |  | $ (3,742) | (3,750) | (3,742) |
Net income (loss) |  |  |  |  |  |  |  |  | (11) | (8) |
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 |
Balance at end of year | (3,761) |  |  |  | (3,750) |  |  |  | (3,761) | (3,750) | $ (3,742)
Net proprietary capital at September 30 | $ 590 |  |  |  | $ 601 |  |  |  | $ 590 | 601 |
Power Program Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Return of power program appropriation investment |  |  |  |  |  |  |  |  |  | $ 10 |

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