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Commitments and Contingencies - Legal Proceedings (Details) $ in Millions | 12 Months Ended
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Sep. 30, 2020 USD ($) Agreements Groups | Sep. 30, 2019 USD ($)
Legal Proceedings |  |
Possible additional future costs for compliance with CCR requirements | $ 949 |
Amount spent under environmental agreements | 280 |
Contribution related to TVARS case | 5 | $ 4
Possible additional future costs for compliance with Clean Water requirements. | 190 |
Amount remaining to be spent under environmental agreements | 11 |
Loss Contingency, Damages Sought, Value | 30 |
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 |
Amount to be invested in certain environmental projects | $ 290 |
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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