Commitments and Contingencies - Legal Proceedings (Details) $ in Millions	12 Months Ended
Sep. 30, 2020 USD ($) Agreements Groups	Sep. 30, 2019 USD ($)
Legal Proceedings
Possible additional future costs for compliance with CCR requirements	$ 949
Amount spent under environmental agreements	280
Contribution related to TVARS case	5	$ 4
Possible additional future costs for compliance with Clean Water requirements.	190
Amount remaining to be spent under environmental agreements	11
Loss Contingency, Damages Sought, Value	30
General
Legal Proceedings
Legal loss contingency accrual	$ 14
Environmental Agreements
Legal Proceedings
Number of similar environmental agreements entered into \| Agreements	2
Number of environmental agreements entered into with the EPA \| Agreements	1
Number of environmental agreements entered into with environmental advocacy groups \| Groups	3
Amount to be invested in certain environmental projects	$ 290
Other long-term liabilities \| General
Legal Proceedings
Legal loss contingency accrual	12
Accounts payable and accrued liabilities \| General
Legal Proceedings
Legal loss contingency accrual	$ 2

Amount remaining to be spent under environmental agreements

No definition available.

Name:	tve_AmountRemainingToBeSpentUnderEnvironmentalAgreements
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount to be invested in certain environmental projects

+ References

No definition available.

+ Details

Name:	tve_AmountToBeInvestedInCertainEnvironmentalProjects
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount spent under environmental agreements

+ References

No definition available.

+ Details

Name:	tve_Amountspentunderenvironmentalagreements
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of environmental agreements entered into with the EPA

+ References

No definition available.

+ Details

Name:	tve_NumberOfEnvironmentalAgreementsEnteredIntoWithEpa
Namespace Prefix:	tve_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of similar environmental agreements entered into

+ References

No definition available.

+ Details

Name:	tve_NumberOfSimilarEnvironmentalAgreementsEnteredInto
Namespace Prefix:	tve_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number environmental advocacy groups with which there is an environmental agreement.

+ References

No definition available.

+ Details

Name:	tve_Numberenvironmentaladvocacygroupswithwhichthereisanenvironmentalagreement.
Namespace Prefix:	tve_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The possible additional future costs for compliance with CCR requirements

+ References

No definition available.

+ Details

Name:	tve_PossibleadditionalfuturecostsforcompliancewithCCRrequirements
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The possible additional future costs for compliance with Clean Water requirements.

+ References

No definition available.

+ Details

Name:	tve_PossibleadditionalfuturecostsforcompliancewithCleanWaterrequirements.
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

+ Details

Name:	us-gaap_LossContingencyAccrualAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14557-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14435-108349

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cash outflow for pension and other postretirement benefits. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3536-108585

+ Details

Name:	us-gaap_PensionAndOtherPostretirementBenefitContributions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	srt_LitigationCaseAxis=tve_GeneralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_LitigationCaseAxis=tve_EnvironmentalAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_AccountsPayableAndAccruedLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: