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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017
Asset Retirement Obligations |  |  |
Increase in ARO liability |  | $ 475 |
Change in estimate | [1] | 430 | $ 411
Additional obligations | [1] | 1 | 1
Amortization of regulatory asset |  | 144 |
Balance | [1] | 4,779 | 4,304
Settlements | [1] | (106) | (123)
Accretion (recorded as regulatory asset) | [1] | 165 | 151
Gain (Loss) on Disposition of Assets | [1] | (15) |
Asset Retirement Obligation, Accretion Expense |  |  |
Non-nuclear - Active Projects Revisions |  |  |
Asset Retirement Obligations |  |  |
Reclassification of Gallatin projects |  |  | 188
Nuclear |  |  |
Asset Retirement Obligations |  |  |
Revision of estimate (other non nuclear) |  | 0 | (250)
Change in estimate |  |  | 250
Additional obligations |  | 0 | 0
Reclassification of Gallatin projects |  |  | 0
Balance |  | 2,989 | 2,859
Settlements |  | 0 | 0
Accretion (recorded as regulatory asset) |  | 130 | 117
Gain (Loss) on Disposition of Assets |  | 0 |
Non-nuclear |  |  |
Asset Retirement Obligations |  |  |
Revision of estimate (other non nuclear) |  | 338 | (188)
Change in estimate |  | 430 | 161
Additional obligations |  | 1 | 1
Balance |  | 1,790 | 1,445
Settlements |  | (106) | (123)
Accretion (recorded as regulatory asset) |  | 35 | 34
Gain (Loss) on Disposition of Assets |  | (15) |
Accounts payable and accrued liabilities |  |  |
Asset Retirement Obligations |  |  |
Current portion of ARO |  | $ 115 | $ 128
[1] | (1) The current portions of the ARO liability in the amounts of $115 million and $128 million as of September 30, 2018 and 2017, respectively, are included in Accounts payable and accrued liabilities.

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- Definition Asset Retirement Obligation, Revision of Estimate (Other) + References No definition available. + Details
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- Definition Reclassification of Gallatin projects + References No definition available. + Details
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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 -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 + Details
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- Definition Current portion of 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of asset retirement obligations incurred during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 35 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of gain (loss) on sale or disposal of assets, including but not limited to property plant and equipment, intangible assets and equity in securities of subsidiaries or equity method investee. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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