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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		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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The amount of accretion recorded as a regulatory asset during the period.

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Asset Retirement Obligation, Revision of Estimate (Other)

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Reclassification of Gallatin projects

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Reconciliation of Asset Retirement Obligations [Line Items]

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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.

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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.

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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.

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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.

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Amount of asset retirement obligations incurred during the period.

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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.

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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.

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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.

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