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Debt and Other Obligations Lease/Leaseback Obligations (Details) $ in Millions | Dec. 31, 2020 USD ($) | Sep. 30, 2020
--- | --- | ---
Sale Leaseback Transaction [Line Items] |  |
CT and QTE outstanding leaseback obligation | $ 223 |
Leasing transaction, number of units |  | 24
CT and QTE outstanding leaseback obligation | $ 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 The number of units involved in leaseback transactions. + References No definition available. + Details
Name: | tve_LeasebackTransactionNumberOfUnits
Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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