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Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Leases |  |  |
Finance lease under PPA | $ 390 | $ 425 |
Operating lease under PPA | 135 | 133 |
Operating cash flows for operating leases | $ 77 | $ 57 | $ 53

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 cash flows for operating leases + References No definition available. + Details
Name: | tve_OperatingCashFlowsForOperatingLeases
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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