Asset Retirement Obligations (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2018	Sep. 30, 2017
Asset Retirement Obligations
Asset Retirement Obligation, Period Increase (Decrease)	$ 85
Amortization and Depreciation of Decontaminating and Decommissioning Assets	72
Balance at September 30, 2017	4,304
Settlements	(42)
Revisions in estimate	45
Asset Retirement Obligation, Liabilities Incurred	1
Accretion (recorded as regulatory asset)	81
Balance at December 31, 2017	4,389
Nuclear
Asset Retirement Obligations
Balance at September 30, 2017	2,859
Settlements	0
Revisions in estimate	0
Asset Retirement Obligation, Liabilities Incurred	0
Accretion (recorded as regulatory asset)	65
Balance at December 31, 2017	2,924
Non-nuclear
Asset Retirement Obligations
Balance at September 30, 2017	1,445
Revisions in estimate	45
Asset Retirement Obligation, Liabilities Incurred	1
Accretion (recorded as regulatory asset)	16
Balance at December 31, 2017	1,465
Accounts payable and accrued liabilities
Asset Retirement Obligations
Current portion of ARO	$ 142	$ 128

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

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- Definition

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.

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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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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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

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