Number of megawatts provided under the power purchase obligations.

No definition available.

Name:	tve_MegawattsProvidedUnderPowerPurchaseObligations
Namespace Prefix:	tve_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Megawatts provided under transmission obligations

+ References

No definition available.

+ Details

Name:	tve_Megawattsprovidedundertransmissionobligations
Namespace Prefix:	tve_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of generation sources under PPAs

+ References

No definition available.

+ Details

Name:	tve_NumberOfGenerationSourcesUnderPPAs
Namespace Prefix:	tve_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The expense for power purchased under contractual power purchase obligations.

+ References

No definition available.

+ Details

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

- Definition

The high end of the range of remaining terms of the agreements.

+ References

No definition available.

+ Details

Name:	tve_Remainingtermsoftheagreementshighendofrange
Namespace Prefix:	tve_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Unfunded loan commitment due in year 2

+ References

No definition available.

+ Details

Name:	tve_Unfundedloancommitmentdueinyear2
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_LongTermPurchaseCommitmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

+ References

No definition available.

+ Details

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

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=tve_PurchaseAgreementsRequiredByFederalLawMember
Namespace Prefix:
Data Type:	na
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Period Type:

Commitments and Contingencies - Purchase Obligations (Details) $ in Millions	12 Months Ended
	Sep. 30, 2023 USD ($) Megawatts megawatts	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2029 USD ($)	Sep. 30, 2028 USD ($)	Sep. 30, 2027 USD ($)	Sep. 30, 2026 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)
Obligations
Megawatts provided under power purchase obligations \| Megawatts	4,487
Remaining terms of the agreements, high end of range	12 years
Megawatts provided under transmission obligations \| megawatts	2,792
Power purchased under agreement \| $	$ 301	$ 318	$ 202
2019 \| $	$ 1
Scenario, Forecast
Obligations
Purchase Obligation \| $				$ 541	$ 154	$ 155	$ 155	$ 155	$ 155
Purchase Agreements Required by Federal Law
Obligations
Megawatts provided under power purchase obligations \| Megawatts	285
Number of generation sources under PPAs \| Megawatts	955