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Commitments and Contingencies Unfunded loan commitments (Details) $ in Millions | Sep. 30, 2019 USD ($)
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Legal Proceedings |
2018 | $ 7
Energy prepayment obligations |
Legal Proceedings |
Prepayment Obligations | $ 10

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- Definition Prepayment Obligations + References No definition available. + Details
Name: | tve_PrepaymentObligations
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Unfunded loan commitment due in next twelve months + References No definition available. + Details
Name: | tve_Unfundedloancommitmentdueinnexttwelvemonths
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=tve_EnergyprepaymentobligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |