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Commitments and Contingencies Commitments and Contingencies - Legal Proceedings (Details) $ in Millions | 12 Months Ended
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Sep. 30, 2019 USD ($) megawatts Units Groups Agreements | Sep. 30, 2018 USD ($)
Legal Proceedings |  |
Possible additional future costs for compliance with CCR requirements | $ 1,181 |
Amount spent under environmental agreements | 279 |
Contribution related to TVARS case | 4 | $ 15
Possible additional future costs for compliance with Clean Water requirements. | 278 |
Amount to Pay Consent Decree | 0 |
General |  |
Legal Proceedings |  |
Legal loss contingency accrual | $ 14 |
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 option 1 | megawatts | 2,200 |
Megawatts option 2 | 3,500 |
Amount to be invested in certain environmental projects | $ 290 |
Amount to be provided to fund environmental projects | 60 |
Amount to pay civil penalties | 10 |
Other long-term liabilities | General |  |
Legal Proceedings |  |
Legal loss contingency accrual | 12 |
Accounts payable and accrued liabilities | General |  |
Legal Proceedings |  |
Legal loss contingency accrual | $ 2 |

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