Asset Retirement Obligations (Details) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2018	Sep. 30, 2017
Asset Retirement Obligations
Asset Retirement Obligation, Period Increase (Decrease)	$ 98
Amortization and Depreciation of Decontaminating and Decommissioning Assets	108
Balance at September 30, 2017	4,304
Settlements	(69)
Revisions in estimate	44
Asset Retirement Obligation, Liabilities Incurred	1
Accretion (recorded as regulatory asset)	122
Balance at June 30, 2018	4,402
Asset Retirement Obligation, Revision of Estimate (Other)	300
Nuclear
Asset Retirement Obligations
Balance at September 30, 2017	2,859
Settlements	0
Revisions in estimate	0
Asset Retirement Obligation, Liabilities Incurred	0
Accretion (recorded as regulatory asset)	97
Balance at June 30, 2018	2,956
Non-nuclear
Asset Retirement Obligations
Balance at September 30, 2017	1,445
Settlements	(69)
Revisions in estimate	44
Asset Retirement Obligation, Liabilities Incurred	1
Accretion (recorded as regulatory asset)	25
Balance at June 30, 2018	1,446
Accounts payable and accrued liabilities
Asset Retirement Obligations
Current portion of ARO	$ 153	$ 128

The amount of accretion recorded as a regulatory asset during the period.

No definition available.

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

- Definition

Asset Retirement Obligation, Revision of Estimate (Other)

+ References

No definition available.

+ Details

Name:	tve_AssetRetirementObligationRevisionofEstimateOther
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reconciliation of Asset Retirement Obligations [Line Items]

+ References

No definition available.

+ Details

Name:	tve_ReconciliationOfAssetRetirementObligationsLineItems
Namespace Prefix:	tve_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The current period expense charged against earnings on decontaminating and decommissioning of facilities used in the normal conduct of business and not intended for resale to allocate or recognize the cost of the assets over their useful lives.

+ References

No definition available.

+ Details

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

- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

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

- Definition

Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

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

- Definition

Amount of asset retirement obligations incurred during the period.

+ References

+ Details

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

- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

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

- Definition

Amount of increase (decrease) in asset retirement obligations.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 35
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846

+ Details

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

- Details

Name:	tve_ReconciliationOfAssetRetirementObligationLiabilityAxis=tve_NuclearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tve_ReconciliationOfAssetRetirementObligationLiabilityAxis=tve_NonNuclearMember
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: