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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Regulatory Assets and Liabilities
Non-current regulatory liabilities	$ 172	$ 40
Current regulatory assets	138	196
Regulatory assets	6,134	7,956
Regulatory Assets	6,272	8,152
Current regulatory liabilities	391	340
Regulatory asset amount expensed	70	72	$ 21
Regulatory Liabilities	563	380
Change in nuclear decommissioning costs	458
Deferred Project Costs [Member]
Regulatory Assets and Liabilities
Gallatin coal combustion residual facilities estimated cost to cap and close	47	114
Regulatory Asset
Regulatory Assets and Liabilities
Regulatory assets	138	150
Removal Costs [Member]
Regulatory Assets and Liabilities
Regulatory assets	479	1,122
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Current regulatory assets	14	3
Regulatory assets	1	0
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Regulatory assets	2,856	2,653
Pension Costs [Member]
Regulatory Assets and Liabilities
Regulatory assets	1,839	3,668
Deferred Fuel Costs [Member]
Regulatory Assets and Liabilities
Current regulatory assets	77	79
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Regulatory assets	821	363
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Non-current regulatory liabilities	102	40
Current regulatory liabilities	173	210
Deferred other post-retirement benefits cost
Regulatory Assets and Liabilities
Non-current regulatory liabilities	70	0
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	$ 218	$ 130

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Regulatory Assets and Liabilities [Line Items]

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The expense charged against earnings during the period to allocate the capitalized costs of regulatory assets over the periods expected to benefit from such costs.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

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