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Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
Leases |  |  |
Finance lease under PPA | $ 550 | $ 390 |
Operating lease under PPA | 121 | 135 |
Operating cash flows for operating leases | 78 | $ 77 | $ 57
Finance Lease, Liability, Payment, Due | $ 1,133 |  |

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- 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

X
- Definition Amount of lessee's undiscounted obligation for lease payments for finance lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478964/842-20-50-6 + Details
Name: | us-gaap_FinanceLeaseLiabilityPaymentsDue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant