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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Liabilities Settled [Line Items] |  |  |  |
Increase in ARO liability |  | $ 327 |  |
Change in estimate | [1] | 369 | $ 247 |
Balance | [1] | 7,489 | 7,162 | $ 7,002
Asset Retirement Obligation, Liabilities Settled | [1] | 279 | 311 |
Accretion (recorded as regulatory asset) | [1] | 237 | 224 |
Asset Retirement Obligation, Current |  | 272 | 275 |
Amortization and Depreciation of Decontaminating and Decommissioning Assets |  | 188 | 137 | 72
Total ARO change due to CCR |  | 458 | 82 |
Change due to cost study |  |  | 58 |
Change in estimate due to coal fines |  |  | 119 |
Change in estimate due to closure of coal yards |  |  | 57 |
Change in estimate due to timing of maintenance |  |  | 53 |
Change in estimate due to timing of asset retirement activities |  |  | 47 |
Change in estimate due to Gallatin CCR closure |  | 60 | 30 |
Change in cost estimates at Allen |  |  | 15 |
Changes in closure liabilities |  | 15 |  |
Increase in Asset Retirement Obligation, Revisions in Estimate |  | 9 |  |
Nuclear |  |  |  |
Liabilities Settled [Line Items] |  |  |  |
Change in estimate |  | 7 | 61 |
Balance |  | 3,808 | 3,643 | 3,428
Asset Retirement Obligation, Liabilities Settled |  | 8 | 2 |
Accretion (recorded as regulatory asset) |  | 166 | 156 |
Non-nuclear |  |  |  |
Liabilities Settled [Line Items] |  |  |  |
Change in estimate |  | 362 | 186 |
Balance |  | 3,681 | 3,519 | $ 3,574
Asset Retirement Obligation, Liabilities Settled |  | 271 | 309 |
Accretion (recorded as regulatory asset) |  | $ 71 | $ 68 |
[1] | (1) Includes $272 million and $275 million at September 30, 2023 and 2022, respectively, in Current liabilities.

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- Definition The current period expense charged against earnings on decontaminating and decommissioning of facilities used in the normal conduct of business and not intended for resale to allocate or recognize the cost of the assets over their useful lives. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-28 + Details
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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 410 -SubTopic 20 -Name Accounting Standards Codification -Section 25 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481999/410-20-25-4 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 410 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481850/410-20-50-1 + Details
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