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Benefit Plans Components of Benefit Plans (Details)	3 Months Ended
	Dec. 31, 2018 plans
Retirement Plan Disclosure
Number of unfunded post-retirement health care plans	2

X

- Definition

The number of unfunded post-retirement health care plans that provide for non-vested contributions toward the cost of certain retirees' medical coverage.

+ References

No definition available.

+ Details

Name:	tve_NumberOfUnfundedPostRetirementHealthCarePlans
Namespace Prefix:	tve_
Data Type:	xbrli:integerItemType
Balance Type:	na
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