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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013
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Derivative |
FTP transaction limit | $ 130
Maximum hedge volume | 75.00%
Market value limitation of outstanding construction materials hedging transactions | 100
Portfolio value at risk limit for foreign currency transactions | $ 5
Swap contracts |
Derivative |
Remaining terms | 5 years

X
- Definition FTP transaction limit + References No definition available. + Details
Name: | tve_FtpTransactionLimit
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The market value limitation of outstanding hedging transactions involving construction materials. + References No definition available. + Details
Name: | tve_MarketValueLimitationOfOutstandingConstructionMaterialsHedgingTransactions
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Definition The maximum hedge volume for construction material transactions. + References No definition available. + Details
Name: | tve_MaximumHedgeVolume
Namespace Prefix: | tve_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The portfolio value at risk limit for foreign currency transactions. + References No definition available. + Details
Name: | tve_PortfolioValueAtRiskLimitForForeignCurrencyTransactions
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Definition Remaining terms + References No definition available. + Details
Name: | tve_RemainingTerms
Namespace Prefix: | tve_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration