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Revenue Economic Development Incentives (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
Revenue from Contract with Customer [Abstract] |  |  |
Revenues | $ 318 | $ 330 | $ 328
Unpaid economic incentives | $ (187) | $ (188) |

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- Definition Paid Economic Incentives + References No definition available. + Details
Name: | tve_PaidEconomicIncentives
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Unpaid economic incentives + References No definition available. + Details
Name: | tve_UnpaidEconomicIncentives
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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