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Debt and Other Obligations Lease/Leaseback Obligations (Details) $ in Millions | Mar. 31, 2019 USD ($) | Sep. 30, 2018
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Debt Disclosure [Abstract] |  |
Leasing transaction, number of units |  | 24
CT and QTE outstanding leaseback obligation | $ 263 |

X
- 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

X
- 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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- References No definition available. + Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration