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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
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

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- Definition

FTP transaction limit

+ 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

The market value limitation of outstanding hedging transactions involving construction materials.

+ References

No definition available.

+ Details

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Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
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- 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

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- Definition

The portfolio value at risk limit for foreign currency transactions.

+ 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

Remaining terms

+ References

No definition available.

+ Details

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Namespace Prefix:	tve_
Data Type:	us-types:durationStringItemType
Balance Type:	na
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- Details

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