Other Long-Term Assets (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2018	Sep. 30, 2017
Other Long-Term Assets
EnergyRight® receivables	$ 119	$ 127
Currency swap assets, net	29
Other long-term assets	370	323
Other long-term assets
Other Long-Term Assets
Loans and other long-term receivables, net	142	115
EnergyRight® receivables	95	100
Prepaid capacity payments	31	34
Commodity contract derivative assets	3	2
Currency swap assets, net	29	0
Other	70	72
Accounts Receivable
Other Long-Term Assets
EnergyRight® receivables	$ 23	$ 25
Energy Right
Other Long-Term Assets
Number of days in default	180 days
Energy Right \| Minimum
Other Long-Term Assets
EnergyRight loan term	5 years
Energy Right \| Maximum
Other Long-Term Assets
EnergyRight loan term	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

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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

Fair value portion of asset contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward contracts, and swaps.

+ 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.

+ References

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 45
-Paragraph 4
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6812-107765

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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