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Risk Management Activities and Derivative Transactions Cash Flow from Hedging (Details) $ in Millions | Mar. 31, 2019 USD ($)
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Cash Flow from Hedging [Abstract] |
Associated bond issues currency exposure | $ 600

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References No definition available. + Details
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Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition Cash Flow from Hedging [Abstract] + References No definition available. + Details
Name: | tve_CashFlowfromHedgingAbstract
Namespace Prefix: | tve_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration