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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)		3 Months Ended
		Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)
Derivative
Amount recognized for unrealized gains (losses)		$ 0
Change in Unrealized gains (losses) on Interest Rate Derivatives		171,000,000	$ 110,000,000
Interest Rate Swap
Derivative
Gain (loss) recognized in income on derivatives	[1]	(21,000,000)	(20,000,000)
Fair value		(436,000,000)		$ (498,000,000)
Commodity Contract Derivatives
Derivative
Gain (loss) recognized in income on derivatives	[1]	1,000,000	$ 0
Fair value		$ (62,000,000)		$ (41,000,000)
Coal Contract Derivatives
Derivative
Number of contracts		9		8
Derivative, Nonmonetary Notional Amount		10,000,000		9,000,000
Fair value		$ (16,000,000)		$ (4,000,000)
Natural gas contract derivatives
Derivative
Number of contracts		53		65
Derivative, Nonmonetary Notional Amount		365,000,000		330,000,000
Fair value		$ (46,000,000)		$ (37,000,000)
[1] All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in income but instead are deferred as regulatory assets and liabilities. As such, there were no related gains (losses) recognized in income for these unrealized gains (losses) for the three months ended December 31, 2019 and 2018.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of interest rate derivatives held at each balance sheet date.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Number of price risk derivative instruments held by the entity at the reporting date.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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