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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | $ 698 | [1] | $ 527
Total redemptions/maturities of debt | [1] | 700 |  |
Repayments of Notes Payable |  | 2 |  | $ 0
electronotes |  |  |  |
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | $ 47 |  |
Interest rate |  | 4.10% |  |
2009 Series B |  |  |  |
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | $ 1 |  |
Interest rate |  | 3.77% |  |
Total |  |  |  |
Debt Instrument |  |  |  |
Percent of par value |  | 100.00% |  |
Notes payable |  |  |  |
Debt Instrument |  |  |  |
Interest rate |  | 1.64% |  |
Repayments of Notes Payable |  | $ 2 |  |
1997 Series E |  |  |  |
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | $ 650 |  |
Interest rate |  | 6.25% |  |
[1] | All redemptions were at 100 percent of par.

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