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Proprietary Capital Proprietary Capital (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Amount of appropriation investment that was repaid | $ 1,000 |  |  |  |  |  |  |  | $ 1,000 |  |
Remaining appropriation investment | 258 |  |  |  |  |  |  |  | 258 |  |
Balance at beginning of year |  |  |  | $ 7,203 |  |  |  | $ 6,104 | 7,203 | $ 6,104 | $ 5,647
Net income (loss) | 661 | $ 291 | $ 318 | (37) | $ 502 | $ 32 | $ 496 | 81 | 1,233 | 1,111 | 469
Return of power program appropriation investment |  |  |  |  |  |  |  |  |  |  | 10
Return on power program appropriation investment |  |  |  |  |  |  |  |  | (6) | (5) | (4)
Balance at end of year | 8,420 |  |  |  | 7,203 |  |  |  | 8,420 | 7,203 | $ 6,104
Net proprietary capital at September 30 | $ 8,420 |  |  |  | $ 7,203 |  |  |  | $ 8,420 | $ 7,203 |
Computed average interest rate payable | 2.04% |  |  |  | 2.04% |  |  |  | 2.04% | 2.04% | 1.97%
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 |  |  |  | 258 | 258 | 258 | 268
Net income (loss) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Return of power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 | 10
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 |  |  |  | 6,357 |  |  |  | 5,240 | 6,357 | 5,240 | 4,767
Net income (loss) |  |  |  |  |  |  |  |  | 1,243 | 1,122 | 477
Return of power program appropriation investment |  |  |  |  |  |  |  |  |  |  | 0
Return on power program appropriation investment |  |  |  |  |  |  |  |  | (6) | (5) | (4)
Balance at end of year | 7,594 |  |  |  | 6,357 |  |  |  | 7,594 | 6,357 | 5,240
Net proprietary capital at September 30 | 7,852 |  |  |  | 6,615 |  |  |  | 7,852 | 6,615 |
Nonpower Programs Retained Earnings |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of year |  |  |  | $ (3,761) |  |  |  | $ (3,750) | (3,761) | (3,750) |
Return on power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 |
Balance at end of year | (3,771) |  |  |  | (3,761) |  |  |  | (3,771) | (3,761) | (3,750)
Net proprietary capital at September 30 | $ 580 |  |  |  | $ 590 |  |  |  | 580 | 590 |
Power Program Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Return of power program appropriation investment |  |  |  |  |  |  |  |  |  |  | 10
Affiliated Entity |  |  |  |  |  |  |  |  |  |  |
Appropriation Investment |  |  |  |  |  |  |  |  |  |  |
Return of power program appropriation investment |  |  |  |  |  |  |  |  | 0 | 0 | 10
Return on power program appropriation investment |  |  |  |  |  |  |  |  | $ (6) | $ (5) | $ (4)

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