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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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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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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Regulatory Assets and Liabilities
Non-current regulatory liabilities	$ 40	$ 23
Current regulatory assets	196	130
Regulatory assets	7,956	10,245
Regulatory Assets	8,152	10,375
Current regulatory liabilities	340	141
Regulatory asset amount expensed	72	21	$ 261
Regulatory Liabilities	380	164
Deferred Project Costs [Member]
Regulatory Assets and Liabilities
Gallatin coal combustion residual facilities estimated cost to cap and close	114	114
Regulatory Asset
Regulatory Assets and Liabilities
Regulatory assets	150	138
Removal Costs [Member]
Regulatory Assets and Liabilities
Regulatory assets	1,122	1,506
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Current regulatory assets	3	4
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Regulatory assets	2,653	2,512
Pension Costs [Member]
Regulatory Assets and Liabilities
Regulatory assets	3,668	5,193
Deferred Fuel Costs [Member]
Regulatory Assets and Liabilities
Current regulatory assets	79	12
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Regulatory assets	363	896
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Non-current regulatory liabilities	40	23
Current regulatory liabilities	210	26
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	$ 130	$ 115