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Revenue Economic Development Incentives (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019
Revenue from Contract with Customer [Abstract] |  |
Revenues | $ 318 | $ 310
Unpaid economic incentives | $ 172 | $ 157

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- Definition Economic incentives + References No definition available. + Details
Name: | tve_Economicincentives
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: | duration

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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