Other Long-Term Assets Other Long-Term Assets (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Other Long-Term Assets
EnergyRight receivables	$ 112	$ 127
Total other long-term assets	$ 349	323
Energy Right
Other Long-Term Assets
Number of days in default	180 days
Accounts Receivable [Member]
Other Long-Term Assets
EnergyRight receivables	$ 22	25
Other long-term assets
Other Long-Term Assets
EnergyRight receivables	90	100
Loans and other long-term receivables, net	135	115
Commodity contract derivative assets	31	2
Prepaid capacity payments	27	34
Other	$ 66	$ 72
Minimum \| Energy Right
Other Long-Term Assets
EnergyRight loan terms	5 years
Maximum \| Energy Right
Other Long-Term Assets
EnergyRight loan terms	10 years

The number of days in default that are determined to be uncollectible.

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- Definition

Other Long-Term Assets [Line Items]

+ References

No definition available.

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- Definition

Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of after a year or beyond the normal operating cycle, if longer.

+ References

+ Details

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

An amount representing an agreement for an unconditional promise by the maker to pay the Entity (holder) a definite sum of money at a future date more than one year from the balance sheet date, net of any write-downs taken for collection uncertainty on the part of the holder. Such amount may include accrued interest receivable in accordance with the terms of the debt. The debt also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. This amount does not include amounts related to receivables held-for-sale.

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- Definition

Amount of other miscellaneous assets expected to be realized or consumed after one year or normal operating cycle, if longer.

+ References

No definition available.

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- Definition

Amount of noncurrent assets classified as other.

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- Definition

Carrying amount as of the balance sheet date of amounts paid in advance which will be charged against earnings in periods after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 210
-SubTopic 10
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- Definition

Carrying amount of financial assets accounted for as secured borrowings recognized in the transferor's statement of financial position at the end of each period presented.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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