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Debt and Other Obligations Debt and Other Obligations - Debt Securities Activity (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014 | Jul. 20, 2016
Debt Instrument |  |  |  |
Issues of power bonds | $ 0 | $ 973,000,000 | $ 989,000,000 |
Discount on debt issues | 0 | (27,000,000) |  |
Acquisition of notes payable |  | 78,000,000 |  |
Redemptions/Maturities of variable interest entities | 33,000,000 | 32,000,000 | 30,000,000 |
Redemptions/Maturities of power bonds | 76,000,000 | 1,180,000,000 | $ 365,000,000 |
Leaseback obligation settled as a result of acquisition |  |  |  | $ 70,000,000
Debt of variable interest entities |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of variable interest entities | 33,000,000 | 32,000,000 |  |
Notes Payable |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of notes payable | 3,000,000 | 0 |  |
Electronotes |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of power bonds | 47,000,000 | 62,000,000 |  |
1998 Series D |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of power bonds | 0 | 50,000,000 |  |
1999 Series A |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of power bonds | 0 | 38,000,000 |  |
2005 Series B |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of power bonds | 0 | 1,000,000,000 |  |
2009 Series A |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of power bonds | 2,000,000 | 3,000,000 |  |
2009 Series B |  |  |  |
Debt Instrument |  |  |  |
Redemptions/Maturities of power bonds | 27,000,000 | 27,000,000 |  |
Total |  |  |  |
Debt Instrument |  |  |  |
Total | $ 112,000,000 | 1,212,000,000 |  |
Percent of par value | 100.00% |  |  |
2015 Series A |  |  |  |
Debt Instrument |  |  |  |
Issues of power bonds | $ 0 | 1,000,000,000 |  |
Percent of par value | 97.31% |  |  |
Total |  |  |  |
Debt Instrument |  |  |  |
Total | $ 0 | 973,000,000 |  |
Notes payable |  |  |  |
Debt Instrument |  |  |  |
Acquisition of notes payable | 78,000,000 | 0 |  |
Total |  |  |  |
Debt Instrument |  |  |  |
Acquisition of notes payable | $ 78,000,000 | $ 0 |  |

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- Definition The amount of debt that an Entity assumes in acquiring a business or in consideration for an asset received in a noncash (or part noncash) acquisition. Noncash is defined as transactions during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
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