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, 2025	Sep. 30, 2024
Derivatives, Fair Value
Amount of letters of credit outstanding	$ 6
Fair value	12	$ 64
250 million Sterling currency swap
Derivatives, Fair Value
Fair value	(47)	(49)
250 million Sterling currency swap \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(43)	(45)
250 million Sterling currency swap \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(4)	(4)
150 million Sterling currency swap
Derivatives, Fair Value
Fair value	(68)	(67)
150 million Sterling currency swap \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(65)	(64)
150 million Sterling currency swap \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(3)	(3)
$1.0 billion notional interest rate swap
Derivatives, Fair Value
Fair value	(526)	(622)
$1.0 billion notional interest rate swap \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(485)	(586)
$1.0 billion notional interest rate swap \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(13)	(10)
$1.0 billion notional interest rate swap \| Interest payable, current
Derivatives, Fair Value
Fair value	(28)	(26)
Commodity contract derivatives
Derivatives, Fair Value
Fair value	10	2
Commodity contract derivatives \| Other Noncurrent Assets [Member]
Derivatives, Fair Value
Fair value	2	2
Commodity contract derivatives \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(4)	(2)
Commodity contract derivatives \| Other current assets
Derivatives, Fair Value
Fair value	14	5
Commodity contract derivatives \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(2)	(3)
$14 million notional
Derivatives, Fair Value
Fair value	(172)	(218)
$14 million notional \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(158)	(206)
$14 million notional \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(5)	(3)
$14 million notional \| Interest payable, current
Derivatives, Fair Value
Fair value	(9)	(9)
Commodity Contract under FHP
Derivatives, Fair Value
Derivative Liability, Fair Value, Gross Liability	85	165
Fair value	(57)	(161)
Commodity Contract under FHP \| Other Noncurrent Assets [Member]
Derivatives, Fair Value
Fair value	8
Commodity Contract under FHP \| Other long-term liabilities
Derivatives, Fair Value
Fair value	(8)	(62)
Commodity Contract under FHP \| Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	$ (57)	$ (99)

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.

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

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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