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Debt and Other Obligations Debt and Other Obligations - Lease/Leasebacks (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2022 | Sep. 30, 2021
Lease/Leasebacks [Abstract] |  |
CT and QTE outstanding leaseback obligation |  | $ 25
Kemper/Lagoon Creek Leasehold Interests |  |
Variable Interest Entities |  |
Leasehold Interests | $ 155 |

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- Definition The outstanding leaseback obligations related to combustion turbine units and qualified technological equipment and software. + References No definition available. + Details
Name: | tve_CtAndQteOutstandingLeasebackObligation
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Lease/Leasebacks [Abstract] + References No definition available. + Details
Name: | tve_LeaseLeasebacksAbstract
Namespace Prefix: | tve_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Leasehold Interests + References No definition available. + Details
Name: | tve_LeaseholdInterests
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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_VariableInterestEntityLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_EquityInterestIssuedOrIssuableByTypeAxis=tve_KemperLagoonCreekLeaseholdInterestsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |