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Benefit Plans Components of Benefit Plans (Details) | 3 Months Ended
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Dec. 31, 2017 plans
Retirement Plan Disclosure |
Number of defined benefit plans | 1
Number of unfunded post-retirement health care plans | 2

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- Definition The number of qualified defined benefit pension plans. + References No definition available. + Details
Name: | tve_NumberOfQualifiedDefinedBenefitPlans
Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- 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

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- 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