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Contingencies and Legal Proceedings Legal Proceedings (Details) | 6 Months Ended |  |
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Mar. 31, 2019 USD ($) megawatts Units Groups Agreements | Sep. 30, 2018 USD ($) | Jun. 30, 2013 USD ($)
Legal Proceedings |  |  |
Number of licensed nuclear units | Units | 7 |  |
Amount paid for remediation under the consent decree |  |  | $ 150,000
Amount paid for remediation under amended consent decree |  | $ 100,000 |
Estimated Litigation Liability | $ 30,000,000 |  |
General |  |  |
Legal Proceedings |  |  |
Legal loss contingency accrual | $ 14,000,000 |  |
Environmental Agreements |  |  |
Legal Proceedings |  |  |
Number of similar environmental agreements entered into | Agreements | 2 |  |
Number of environmental agreements entered into with the EPA | Agreements | 1 |  |
Number of environmental agreements entered into with environmental advocacy groups | Groups | 3 |  |
Number of units to be idled | Units | 18 |  |
Megawatts | megawatts | 2,200 |  |
Megawatts option 2 | 3,500 |  |
Amount to be invested in certain TVA environmental projects | $ 290,000,000 |  |
Amount to be provided to fund environmental projects | 60,000,000 |  |
Amount to pay civil penalties | 10,000,000 |  |
Other long-term liabilities | General |  |  |
Legal Proceedings |  |  |
Legal loss contingency accrual | 12,000,000 |  |
Accounts payable and accrued liabilities | General |  |  |
Legal Proceedings |  |  |
Legal loss contingency accrual | $ 2,000,000 |  |

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