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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Reclassificatons (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Reclassifications |  |  |
Restricted Cash |  | $ 12 | $ 1
Defined Benefit Plan, Other Cost (Credit) | $ 258 | 256 | 758
Reclass of Other, net |  | (22) | (60)
Reclass of Other regulatory amortization and deferral |  | 2 | 143
Other regulatory amortization and deferrals [Member] |  |  |
Reclassifications |  |  |
Deferred Fuel Cost |  | (30) | 98
Reclass of fuel cost tax equivalents |  | (7) | 5
Reclass of Other, net |  | 39 | 40
Other, net [Member] |  |  |
Reclassifications |  |  |
Reclass of accounts payable |  | (9) | (10)
Regulatory assets costs |  | (13) | (50)
Payments to U S Treasury |  | (5) | (5)
Reclass of short-term debt issues (redemptions) |  | (29) | $ (9)
Other Long-Term Assets |  |  |
Reclassifications |  |  |
Restricted Cash |  | $ 13 |

X
- Definition Fuel cost adjustment deferral + References No definition available. + Details
Name: | tve_Fuelcostadjustmentdeferral
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Payments to U.S. Treasury in repayment of, and as return on, the government's appropriation investment. + References No definition available. + Details
Name: | tve_PaymentsToUSTreasury
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Reclassification of restricted cash + References No definition available. + Details
Name: | tve_Reclassificationofrestrictedcash
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Reclassifications [Line Items] + References No definition available. + Details
Name: | tve_ReclassificationsLineItems
Namespace Prefix: | tve_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reclass of Other, net + References No definition available. + Details
Name: | tve_ReclassofOthernet
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Reclass of Other regulatory amortization and deferral + References No definition available. + Details
Name: | tve_ReclassofOtherregulatoryamortizationanddeferral
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Reclass of accounts payable + References No definition available. + Details
Name: | tve_Reclassofaccountspayable
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Reclass of fuel cost tax equivalents + References No definition available. + Details
Name: | tve_Reclassoffuelcosttaxequivalents
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Reclass of regulatory assets costs + References No definition available. + Details
Name: | tve_Reclassofregulatoryassetscosts
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Reclass of short-term debt issues (redemptions) + References No definition available. + Details
Name: | tve_Reclassofshorttermdebtissuesredemptions
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of defined benefit plan cost (credit), classified as other. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanOtherCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | tve_StatementofCashFlowsAxis=tve_OtherregulatoryamortizationanddeferralsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | tve_StatementofCashFlowsAxis=tve_OthernetMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=tve_OtherLongTermAssetsDomain
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |