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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions | 6 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Debt Instrument |  |  |  |
Redemptions/maturities of power bonds |  | $ 1,003 | [1] | $ 700
Total redemptions/maturities of debt | [1] | (1,043) |  |
Notes Issued | [1] | $ 21 |  |
Total |  |  |  |
Debt Instrument |  |  |  |
Percent of par value |  | 100.00% |  |
[1] | (1) All redemptions were at 100 percent of par.

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