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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Other Derivative Instruments (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Sep. 30, 2017
--- | --- | ---
Derivative |  |
Forward Contract Derivative Asset, at Fair Value | $ 45 | $ 19
Fair Value, Inputs, Level 2 |  |
Derivative |  |
Forward Contract Derivative Asset, at Fair Value | $ 45 | $ 19

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- Definition Fair value as of the balance sheet date of forward contract derivative assets. + References No definition available. + Details
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Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |