Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Sep. 30, 2015
Regulatory Assets and Liabilities
Current regulatory assets	$ 503	$ 506
Non-current regulatory assets	10,237	10,418
Regulatory assets	10,740	10,924
Current regulatory liabilities	182	164
Non-current regulatory liabilities	4	2
Regulatory liabilities	186	166
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	157	164
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Non-current regulatory liabilities	4	2
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory liabilities	25	0
Deferred nuclear generating units
Regulatory Assets and Liabilities
Current regulatory assets	237	237
Non-current regulatory assets	1,004	1,042
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory assets	169	162
Non-current regulatory assets	81	63
Environmental agreements
Regulatory Assets and Liabilities
Current regulatory assets	49	47
Non-current regulatory assets	41	55
Environmental cleanup costs - Kingston ash spill
Regulatory Assets and Liabilities
Current regulatory assets	43	43
Non-current regulatory assets	331	348
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory assets	3	15
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	5,556	5,565
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Non-current regulatory assets	1,147	1,236
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	970	1,003
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	835	828
Other non-current regulatory assets
Regulatory Assets and Liabilities
Non-current regulatory assets	$ 272	$ 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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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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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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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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