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Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details) $ in Millions | 12 Months Ended
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Sep. 30, 2015 USD ($) Megawatts Hours Suppliers Facilities | Sep. 30, 2014 USD ($) | Sep. 30, 2013 USD ($)
Obligations |  |  |
Megawatts provided under power purchase obligations | Megawatts | 1,255 |  |
Remaining terms of the agreements, high end of range | 17 years |  |
Power purchased under agreement | $ 230 | $ 237 | $ 267
Number of U.S. Army Corps of Engineers hydroelectric facilities contracted with to obtain power | Facilities | 8 |  |
Agreement termination notice period | 3 years |  |
Minimum required hours of power | Hours | 1,500 |  |
Megawatt allocation | Megawatts | 405 |  |
Maximum term length for the purchase and transportation of coal | 5 years |  |
Maximum term for purchase of enriched uranium and fabrication of nuclear fuel assemblies | 15 years |  |
Purchase Agreements Required by Federal Law |  |  |
Obligations |  |  |
Megawatts provided under power purchase obligations | Megawatts | 882 |  |
Number of suppliers | Suppliers | 24 |  |
Purchase Obligations - Coal |  |  |
Obligations |  |  |
Long-term purchase commitment | $ 1,500 |  |
Purchase Obligations - Nuclear Fuel |  |  |
Obligations |  |  |
Long-term purchase commitment | 3,400 |  |
Purchase Obligations - Other |  |  |
Obligations |  |  |
Long-term purchase commitment | $ 2,845 |  |

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- Definition The maximum term for purchase of enriched uranium and fabrication of nuclear fuel assemblies. + References No definition available. + Details
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- Definition Maximum term length for the purchase and transportation of coal + References No definition available. + Details
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- Definition Number of megawatts provided under the power purchase obligations. + References No definition available. + Details
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- Definition Minimum required hours of power + References No definition available. + Details
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- Definition Number of U.S. Army Corps of Engineers hydroelectric facilities contracted with to obtain power + References No definition available. + Details
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- Definition The high end of the range of remaining terms of the agreements. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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