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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | $ 1,002 | [1] | $ 698
Total redemptions/maturities of debt | [1] | (1,003) |  |
Repayments of Notes Payable |  | 1 |  | $ 2
2013 Series A [Member] |  |  |  |
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | 1,000 |  |
2009 Series B [Member] |  |  |  |
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | 1 |  |
Notes Payable [Member] |  |  |  |
Debt Instrument |  |  |  |
Repayments of Notes Payable |  | 1 |  |
electronotes [Member] |  |  |  |
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | $ 1 |  |
Total |  |  |  |
Debt Instrument |  |  |  |
Percent of par value |  | 100.00% |  |
[1] | (1) All redemptions were at 100 percent of par.

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