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Contingencies and Legal Proceedings Legal Proceedings (Details) $ in Millions	9 Months Ended
	Jun. 30, 2018 USD ($) People megawatts Groups Units Years Agreements
General
Legal Proceedings
Legal loss contingency accrual	$ 19
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
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 from TVA	$ 1,000
Retirement age of eligibility for cost of living adjustment before January 1, 2010 | Years	55
Retirement age of eligibility for cost of living adjustment after January 1, 2010 | Years	60
Other long-term liabilities | General
Legal Proceedings
Legal loss contingency accrual	$ 11
Accounts payable and accrued liabilities | General
Legal Proceedings
Legal loss contingency accrual	$ 8

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Number of similar environmental agreements entered into

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