Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Regulatory Assets and Liabilities
Current regulatory assets	$ 380	$ 414
Non-current regulatory assets	6,962	6,612
Regulatory assets	7,342	7,026
Current regulatory liabilities	194	187
Non-current regulatory liabilities	95	104
Regulatory liabilities	289	291
Deferred other post-retirement benefits cost
Regulatory Assets and Liabilities
Non-current regulatory liabilities	68	73
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	144	146
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory liabilities	50	41
Non-current regulatory liabilities	27	31
Gallatin coal combustion residual facilities
Regulatory Assets and Liabilities
Gallatin coal combustion residual facilities estimated cost to cap and close	29	38
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Current regulatory assets	75	73
Non-current regulatory assets	800	692
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory assets	5	4
Non-current regulatory assets	5	8
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory assets	75	30
Deferred nuclear generating units
Regulatory Assets and Liabilities
Current regulatory assets	1	3
Non-current regulatory assets	12	11
Environmental cleanup costs – Kingston ash spill
Regulatory Assets and Liabilities
Current regulatory assets	195	266
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	866	861
Deferred pension costs and other post-retirement benefits costs
Regulatory Assets and Liabilities
Non-current regulatory assets	3,059	3,119
Gallatin coal combustion residual facilities
Regulatory Assets and Liabilities
Non-current regulatory assets	1,006	784
Environmental agreements
Regulatory Assets and Liabilities
Non-current regulatory assets	1,086	1,019
Other non-current regulatory assets
Regulatory Assets and Liabilities
Non-current regulatory assets	$ 128	$ 118

Gallatin coal combustion residual facilities estimated cost to cap and close

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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://fasb.org/us-gaap/role/ref/legacyRef
-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.

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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://fasb.org/us-gaap/role/ref/legacyRef
-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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- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
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