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Commitments and Contingencies Commitments and Contingencies - Energy Prepayment Obligations (Details) $ in Millions	Sep. 30, 2018 USD ($)
Interest Payments relating to energy prepayment obligations
Obligations
2019	$ 4

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- Definition

Amount of contractual obligation due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

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

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- Details

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