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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions | 6 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2017
Debt Instrument |  |  |  |  |  |  |
Long-term notes payable |  | $ 1 |  |  |  |  |
Total long-term debt, net |  | 18,439 |  |  | $ 20,183 |  |
Redemptions and repurchases of power bonds |  | $ 1,218 | [1] | $ 1,003 |  |  |
Variable Interest Entity, Financial or Other Support, Type |  | 20 |  |  |  |  |
Total redemptions/maturities of debt | [1] | $ (1,238) |  |  |  |  |
electronotes [Member] |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions and repurchases of power bonds |  | 217 |  |  |  |  |
2009 Series B [Member] |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions and repurchases of power bonds |  | $ 1 |  | $ 1,000 |  |  |
Total |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Percent of par value |  | 100.00% |  |  |  |  |
Debt of variable interest entities [Member] |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Other Debt instrument, Interest Rate, Effective Percentage |  |  |  |  |  | 4.00% |
electronotes [Member] |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 3.00% |  |  |  |  |
2009 Series B [Member] |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 4.00% |  |  |  |  | 2.00%
[1] | (1) Amounts for notes payable were less than $1 million at March 31, 2020, and are therefore not represented in the table above.(2) All redemptions were at 100 percent of par.

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