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Debt and Other Obligations Debt and Other Obligations - Debt Securities Activity (Details) $ in Millions |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 20, 2017 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Feb. 07, 2017 USD ($) | Jul. 20, 2016 USD ($) | Sep. 30, 2003 Units
Debt Instrument |  |  |  |  |  |  |
Number of Businesses Acquired | 2 |  |  |  |  |  |
Leaseback transaction, number of units | Units |  |  |  |  |  |  | 24
Issues of power bonds |  | $ 998 | $ 999 | $ 0 |  |  |
Discount on debt issues |  |  | (1) |  | $ (2) |  |
Acquisition of notes payable |  |  | 74 | 78 |  |  |
Redemptions/Maturities of variable interest entities |  | 36 | 35 | 33 |  |  |
Redemptions/Maturities of power bonds |  | $ 1,731 | $ 1,558 | $ 76 |  |  |
Leaseback obligation settled as a result of acquisition | $ 70 |  |  |  |  | $ 70 |
Total |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Debt Instrument, Redemption Period, End Date |  | 1820 | 1620 |  |  |  |
Percent of par value |  | 100.00% |  |  |  |  |
Debt of variable interest entities |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of variable interest entities |  | $ 36 | $ 35 |  |  |  |
Notes Payable |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of notes payable |  | 53 | 27 |  |  |  |
electronotes |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of power bonds |  | 52 | 5 |  |  |  |
2009 Series B |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of power bonds |  | 29 | 28 |  |  |  |
2001 Series D |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of power bonds |  | 0 | 525 |  |  |  |
2007 Series A |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of power bonds |  | 0 | 1,000 |  |  |  |
1997 Series E [Member] |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of power bonds |  | 650 | 0 |  |  |  |
2008 Series B [Member] |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Redemptions/Maturities of power bonds |  | 1,000 | 0 |  |  |  |
Notes payable |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Acquisition of notes payable |  | 0 | 74 |  |  |  |
Total |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Debt Securities Issues |  | 998 | 999 |  |  |  |
2017 Series A |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |
Issues of power bonds |  | $ 0 | $ 1,000 |  |  |  |
Percent of par value |  | 99.80% |  |  |  |  |

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