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Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Regulatory Assets and Liabilities
Current regulatory assets	$ 471	$ 447
Non-current regulatory assets	8,240	8,698
Regulatory assets	8,711	9,145
Current regulatory liabilities	159	163
Non-current regulatory liabilities	19	25
Regulatory liabilities	178	188
Deferred other post-retirement benefits cost
Regulatory Assets and Liabilities
Non-current regulatory liabilities	16	23
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	147	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	12	8
Non-current regulatory liabilities	3	2
Deferred nuclear generating units
Regulatory Assets and Liabilities
Current regulatory assets	237	237
Non-current regulatory assets	645	759
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Current regulatory assets	80	93
Non-current regulatory assets	824	982
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory assets	44	68
Non-current regulatory assets	36	9
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory assets	39	1
Environmental agreements
Regulatory Assets and Liabilities
Current regulatory assets	3	2
Non-current regulatory assets	11	13
Environmental cleanup costs - Kingston ash spill
Regulatory Assets and Liabilities
Current regulatory assets	44	44
Non-current regulatory assets	242	263
Gallatin coal combustion residual facilities
Regulatory Assets and Liabilities
Current regulatory assets	21	0
Non-current regulatory assets	878	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	3,881	4,009
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	821	823
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	701	703
Other non-current regulatory assets
Regulatory Assets and Liabilities
Non-current regulatory assets	$ 201	$ 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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- 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

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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