Other Long-Term Assets (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2019	Sep. 30, 2018
Other Long-Term Assets
EnergyRight® receivables	$ 106	$ 112
Other long-term assets	347	362
Other long-term assets
Other Long-Term Assets
Loans and other long-term receivables, net(1)	144	125
EnergyRight® receivables	86	90
Prepaid capacity payments	23	27
Commodity contract derivative assets	10	31
Other	61	66
Restricted Cash and Cash Equivalents, Noncurrent	23	23
Accounts Receivable
Other Long-Term Assets
EnergyRight® receivables	$ 21	$ 22
Energy Right
Other Long-Term Assets
Number of days in default	180 days
Energy Right \| Minimum
Other Long-Term Assets
Debt Instrument, Term	5 years
Energy Right \| Maximum
Other Long-Term Assets
Debt Instrument, Term	10 years

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

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Other Long-Term Assets [Line Items]

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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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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

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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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Amount of noncurrent assets classified as other.

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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.

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- Definition

Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as noncurrent. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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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.

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