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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2015
Asset Retirement Obligations
Increase in ARO liability		$ 209
ARO-Nuclear decommissioning study update		5 years
Amortization of regulatory asset		$ 144	$ 44
Balance	[1]	3,843	3,169
Settlements	[1]	(133)	(58)
Change in estimate	[1]	176	480
Additional obligations	[1]	15	101
Accretion (recorded as regulatory asset)	[1]	151	151
Balance	[1]	4,052	3,843
Current portion of ARO		212	161
Nuclear
Asset Retirement Obligations
Balance		2,187	2,052
Settlements		0	0
Change in estimate		198	36
Additional obligations		0	0
Revision of estimate (sequoyah license)	[1]		36
Accretion (recorded as regulatory asset)		107	99
Balance		2,492	2,187
Non-nuclear - Closure Method Revision
Asset Retirement Obligations
Change in estimate		32
Non-nuclear - Active Projects Revisions
Asset Retirement Obligations
Change in estimate		(54)
Non-nuclear
Asset Retirement Obligations
Balance		1,656	1,117
Settlements		(133)	(58)
Change in estimate		(22)	444
Additional obligations		15	101
Revision of estimate (EPA ruling)			469
Revision of estimate (other non nuclear)			25
Accretion (recorded as regulatory asset)		44	52
Balance		$ 1,560	1,656
Plant acquisition | Non-nuclear
Asset Retirement Obligations
Additional obligations			7
Coal-fired obligations | Non-nuclear
Asset Retirement Obligations
Additional obligations			$ 94
[1] The current portions of the ARO liability in the amounts of $212 million and $161 million as of September 30, 2016 and 2015, respectively, are included in Accounts payable and accrued liabilities.

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- Definition

The maximum number of years in between ARO-nuclear decommissioning study updates.

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The amount of accretion recorded as a regulatory asset during the period.

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- Definition

Amount of the Increase of Decrease in the amount of the asset retirement obligation as a result of the EPA ruling during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

No definition available.

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Amount of the Increase or Decrease in the amount of the asset retirement obligation as a result of the nuclear license extension during the current period for changes in the amount of timing of the estimated cash flows associated with the settlement of the obligation.

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Asset Retirement Obligation, Revision of Estimate (Other)

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Reconciliation of Asset Retirement Obligations [Line Items]

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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.

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No definition available.

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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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
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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.

+ References

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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 asset retirement obligations.

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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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