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Summary of Significant Accounting Policies - General (Details) $ in Millions	12 Months Ended
	Sep. 30, 2023 USD ($) People	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2016 USD ($)
Accounting Policies [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 501	$ 500
Reimbursements from DOE	219
Restricted Cash and Investments, Noncurrent	20	20
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	521	520	$ 518	$ 521
Accounts receivable from DOE	11
Appropriation-investment power program	$ 258	258			$ 1,000
Population of Service Area [Line Items]
Population of service area | People	10,000,000
Cash and Cash Equivalents, at Carrying Value	$ 501	500
Restricted Cash and Investments, Noncurrent	$ 20	$ 20

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Accounts receivable

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Balance of appropriated funds advanced and property transferred to/from the reporting entity by the U.S. government as an investment in its power program.

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The population of the company's service area.

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Population of Service Area [Line Items]

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Reimbursements from DOE

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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Amount of cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage. Excludes amount for disposal group and discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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The noncurrent cash, cash equivalents and investments that is restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits classified as long-term; that is not expected to be released from such existing restrictions within one year of the balance sheet date or operating cycle, whichever is longer. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes noncurrent cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

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