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v3.20.2

Summary of Significant Accounting Policies - Leases (Details) $ in Millions | Sep. 30, 2020 USD ($)
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Leases |
Finance lease under PPA | $ 500
Operating lease under PPA | $ 174

X
- Definition Finance lease under PPA + References No definition available. + Details
Name: | tve_FinanceLeaseUnderPPA
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Leases [Line Items] + References No definition available. + Details
Name: | tve_LeasesLineItems
Namespace Prefix: | tve_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Operating lease under PPA + References No definition available. + Details
Name: | tve_OperatingLeaseUnderPPA
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant