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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2017
Debt Instrument |  |  |  |  |  |  |  |  |
Repayments of Other Long-term Debt |  |  |  |  |  | 19 |  |  |
Foreign Currency Transaction Gain (Loss), before Tax |  | $ 167 | $ 147 |  |  |  |  |  |
Total long-term debt, net |  | $ 20,223 | $ 21,307 |  |  | $ 20,223 |  |  |
Redemptions/maturities of power bonds |  |  |  |  |  | 1,034 | [1] | $ 1,731 |
Total redemptions/maturities of debt | [1] |  |  |  |  | (1,099) |  |  |
Notes Issued |  |  |  |  |  | 46 |  |  |
Notes Payable [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  | 1.27% |  |  |  | 1.27% |
electronotes [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Redemptions/maturities of power bonds |  |  |  | $ 2 | $ 1 | 1 |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  | 1.75% |  |  |  | 1.75% |
2009 Series B [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Redemptions/maturities of power bonds |  |  |  |  |  | 1 |  | $ 29 |
2013 Series A [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Redemptions/maturities of power bonds |  |  |  |  |  | $ 1,000 |  |  |
Total |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Percent of par value |  | 100.00% |  |  |  | 100.00% |  |  |
Debt of variable interest entities [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Other Debt instrument, Interest Rate, Effective Percentage |  |  |  | 4.31% |  |  |  | 4.31% |
2018 Series A [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  | 2.89% | 3.48% |  |  | 2.89% |
1997 Series E [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  |  |  |  |  |  | 3.77%
electronotes [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 2.65% |  |  |  | 2.65% |  |  |
2009 Series B [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  |  |  |  |  |  | 3.77%
[1] | (1) All redemptions were at 100 percent of par.

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