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Debt and Other Obligations Debt and Other Obligations - Lease/Leasebacks (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Sep. 30, 2020
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Lease/Leasebacks [Abstract] |  |
CT and QTE outstanding leaseback obligation | $ 25 | $ 223

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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