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Benefit Plans Sensitivity to Certain Changes in Pension Assumptions (Details) $ in Millions	12 Months Ended
	Sep. 30, 2016 USD ($)
Discount rate
Defined Benefit Plan Disclosure
Change in Assumption	(0.25%)
Impact on Pension Cost	$ 22
Impact on Projected Benefit Obligation	$ 388
Rate of return on plan assets
Defined Benefit Plan Disclosure
Change in Assumption	(0.25%)
Impact on Pension Cost	$ 16

X

- Definition

The change in the discount date or the rate of return on plan assets assumption that is used in disclosure of sensitivity of the net periodic pension cost and the projected benefit obligation to changes in those rates.

+ References

No definition available.

+ Details

Name:	tve_ChangeInDiscountRateOrRateOfReturnOnPlanAssetsAssumption
Namespace Prefix:	tve_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Amount of effect of change in rate on the net periodic benefit cost for pension benefits.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of effect of change in rate on the projected benefit obligation for pension benefits.

+ References

No definition available.

+ Details

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

X

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

X

- Details

Name:	tve_PensionBenefitRatesAxis=tve_PensionPlanDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	tve_PensionBenefitRatesAxis=tve_PensionBenefitExpectedRateOfReturnOnPlanAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: