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v3.25.3

Revenue (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Electric revenue | $ (13,483) | $ (12,123) | $ (11,888)
Pandemic Relief Credit | $ 225 |  |
Revenue from Contract with Customer [Abstract] |  |  |
Percent of Pandemic Credit Offered | 2.50% |  |

X
- Definition Revenue from sales of electricity + References No definition available. + Details
Name: | tve_Electricrevenue
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Pandemic Relief Credit + References No definition available. + Details
Name: | tve_PandemicReliefCredit
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percent of Pandemic Credit Offered + References No definition available. + Details
Name: | tve_PercentOfPandemicCreditOffered
Namespace Prefix: | tve_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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