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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, 2018 USD ($)	Dec. 31, 2017 USD ($)		Sep. 30, 2018 USD ($)
Derivative
Amount recognized for unrealized gains (losses)		$ 0
Change in Unrealized gains (losses) on Interest Rate Derivatives		(110,000,000)	$ 28,000,000
Interest Rate Swap
Derivative
Gain (loss) recognized in income on derivatives		(20,000,000)	(24,000,000)	[1]
Fair value		(343,000,000)			$ (317,000,000)
Commodity Contract Derivatives
Derivative
Gain (loss) recognized in income on derivatives		0	(3,000,000)
Fair value		67,000,000			$ 60,000,000
Commodity derivatives under the financial trading program
Derivative
Gain (loss) recognized in income on derivatives	[1]	$ 0	$ 8,000,000
Coal Contract Derivatives
Derivative
Number of contracts		16			13
Notional amount		17,000,000			20,000,000
Fair value		$ 63,000,000			$ 58,000,000
Natural gas contract derivatives
Derivative
Number of contracts		53			61
Notional amount		347,000,000			359,000,000
Fair value		$ 4,000,000			$ 2,000,000
[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 was no related gain (loss) recognized in income for these unrealized gains (losses) for the three months ended December 31, 2018 and 2017.

X

- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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-Name Accounting Standards Codification
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-Section 50
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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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