Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Mark-to-Market Values of TVA Derivatives (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Derivatives, Fair Value
Amount of letters of credit outstanding	$ 6
Fair value	52	$ 1
250 million Sterling currency swap
Derivatives, Fair Value
Fair value	(72)	(130)
250 million Sterling currency swap \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(66)	(123)
250 million Sterling currency swap \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(6)	(7)
150 million Sterling currency swap
Derivatives, Fair Value
Fair value	(69)	(110)
150 million Sterling currency swap \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(65)	(105)
150 million Sterling currency swap \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(4)	(5)
$1.0 billion notional interest rate swap
Derivatives, Fair Value
Fair value	(499)	(672)
$1.0 billion notional interest rate swap \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(473)	(630)
$1.0 billion notional interest rate swap \| Other current assets
Derivatives, Fair Value
Fair value	1
$1.0 billion notional interest rate swap \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value		(9)
$1.0 billion notional interest rate swap \| Interest payable, current
Derivatives, Fair Value
Fair value	(27)	(33)
Commodity contract derivatives
Derivatives, Fair Value
Fair value	31	145
Commodity contract derivatives \| Other Noncurrent Assets [Member]
Derivatives, Fair Value
Fair value	12	34
Commodity contract derivatives \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(1)	(1)
Commodity contract derivatives \| Other current assets
Derivatives, Fair Value
Fair value	21	118
Commodity contract derivatives \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(1)	(6)
$14 million notional
Derivatives, Fair Value
Fair value	(159)	(233)
$14 million notional \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(154)	(221)
$14 million notional \| Other current assets
Derivatives, Fair Value
Fair value	3
$14 million notional \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value		(3)
$14 million notional \| Interest payable, current
Derivatives, Fair Value
Fair value	(8)	(9)
Commodity Contract under FHP
Derivatives, Fair Value
Derivative Liability, Fair Value, Gross Liability	212
Fair value	(186)	115
Commodity Contract under FHP \| Other Noncurrent Assets [Member]
Derivatives, Fair Value
Fair value		68
Commodity Contract under FHP \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(51)
Commodity Contract under FHP \| Other current assets
Derivatives, Fair Value
Fair value		54
Commodity Contract under FHP \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	$ (135)	(8)
Commodity Contract under FHP \| Accounts Receivable [Member]
Derivatives, Fair Value
Fair value		$ 1

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483444/210-20-55-22

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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-Name Accounting Standards Codification
-Section 50
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
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-Topic 860
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481326/860-20-50-4D

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

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
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