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Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Regulatory Assets and Liabilities
Current regulatory assets	$ 455	$ 447
Non-current regulatory assets	8,492	8,698
Regulatory assets	8,947	9,145
Current regulatory liabilities	159	163
Non-current regulatory liabilities	25	25
Regulatory liabilities	184	188
Deferred other post-retirement benefits cost
Regulatory Assets and Liabilities
Non-current regulatory liabilities	19	23
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	149	153
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory liabilities	0	2
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory liabilities	10	8
Non-current regulatory liabilities	6	2
Deferred nuclear generating units
Regulatory Assets and Liabilities
Current regulatory assets	237	237
Non-current regulatory assets	703	759
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Current regulatory assets	89	93
Non-current regulatory assets	957	982
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory assets	57	68
Non-current regulatory assets	35	9
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory assets	12	1
Environmental agreements
Regulatory Assets and Liabilities
Current regulatory assets	3	2
Non-current regulatory assets	12	13
Environmental cleanup costs - Kingston ash spill
Regulatory Assets and Liabilities
Current regulatory assets	44	44
Non-current regulatory assets	253	263
Gallatin coal combustion residual facilities
Regulatory Assets and Liabilities
Current regulatory assets	10	0
Non-current regulatory assets	889	899
Other current regulatory assets
Regulatory Assets and Liabilities
Current regulatory assets	3	2
Deferred pension costs and other post-retirement benefits costs
Regulatory Assets and Liabilities
Non-current regulatory assets	3,945	4,009
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	771	823
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	692	703
Other non-current regulatory assets
Regulatory Assets and Liabilities
Non-current regulatory assets	$ 235	$ 238

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- Definition

Regulatory Assets and Liabilities [Line Items]

+ References

No definition available.

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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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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
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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
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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.

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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 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)
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