Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2016	Sep. 30, 2015
Regulatory Assets and Liabilities
Current regulatory assets	$ 604	$ 506
Non-current regulatory assets	10,424	10,418
Regulatory assets	11,028	10,924
Current regulatory liabilities	149	164
Non-current regulatory liabilities	4	2
Regulatory liabilities	153	166
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	141	164
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory liabilities	8	0
Non-current regulatory liabilities	4	2
Deferred nuclear generating units
Regulatory Assets and Liabilities
Current regulatory assets	237	237
Non-current regulatory assets	901	1,042
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory assets	160	162
Non-current regulatory assets	87	63
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory assets	119	15
Environmental agreements
Regulatory Assets and Liabilities
Current regulatory assets	43	47
Non-current regulatory assets	25	55
Environmental cleanup costs - Kingston ash spill
Regulatory Assets and Liabilities
Current regulatory assets	43	43
Non-current regulatory assets	309	348
Other current regulatory assets
Regulatory Assets and Liabilities
Current regulatory assets	2	2
Deferred pension costs and other post-retirement benefits costs
Regulatory Assets and Liabilities
Non-current regulatory assets	5,541	5,565
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Non-current regulatory assets	1,521	1,236
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	972	1,003
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	810	828
Other non-current regulatory assets
Regulatory Assets and Liabilities
Non-current regulatory assets	$ 258	$ 278

Regulatory Assets and Liabilities [Line Items]

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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

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- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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- Definition

The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

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- Definition

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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