Commitments and Contingencies Commitments and Contingencies - Legal Proceedings (Details) $ in Millions	12 Months Ended
Sep. 30, 2018 USD ($) People Units megawatts Groups Agreements	Sep. 30, 2017 USD ($)
Legal Proceedings
Possible additional future costs for compliance with CCR requirements	$ 1,178
Amount spent under environmental agreements	276
Contribution related to TVARS case	15	$ 20
Possible additional future costs for compliance with Clean Water requirements.	466
Amount to Pay Consent Decree	0
General
Legal Proceedings
Legal loss contingency accrual	$ 18
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
Case Involving Tennessee Valley Authority Retirement System
Legal Proceedings
Number of participants that filed suit \| People	8
Contribution related to TVARS case	$ 1,000
Other long-term liabilities \| General
Legal Proceedings
Legal loss contingency accrual	11
Accounts payable and accrued liabilities \| General
Legal Proceedings
Legal loss contingency accrual	$ 7

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Amount to be provided to fund environmental projects in Alabama, Kentucky, North Carolina, and Tennessee

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Amount to pay civil penalties

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Amount spent under environmental agreements

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Megawatts of summer net capability

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Megawatts option 2

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Number of environmental agreements entered into with the EPA

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The number of participants that filed suit.

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Number of similar environmental agreements entered into

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Number of units to be idled.

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Number environmental advocacy groups with which there is an environmental agreement.

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The possible additional future costs for compliance with CCR requirements

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The possible additional future costs for compliance with Clean Water requirements.

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Amount of loss contingency liability.

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Amount of cash outflow for pension and other postretirement benefits. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree.

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