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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivatives Under Financial Trading Program (Details) - Natural Gas Contract Derivatives - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018
Derivative |  |
Decrease (increase) in fuel expense | $ 0 | $ (6)
Decrease (increase) in purchased power expense | $ 0 | $ (2)

X
- Definition The decrease (increase) in fuel expense relating to the FTP. + References No definition available. + Details
Name: | tve_DecreaseIncreaseInFuelExpense
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition The decrease (increase) in purchased power expense related to the FTP. + References No definition available. + Details
Name: | tve_DecreaseIncreaseInPurchasedPowerExpense
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | tve_DerivativesUnderFinancialTradingProgramAxis=srt_NaturalGasReservesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |