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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014
Derivative
FTP transaction limit	130tve_FtpTransactionLimit
Maximum hedge volume	75.00%tve_MaximumHedgeVolume
Market value limitation of outstanding construction materials hedging transactions	100tve_MarketValueLimitationOfOutstandingConstructionMaterialsHedgingTransactions
Portfolio value at risk limit for foreign currency transactions	5tve_PortfolioValueAtRiskLimitForForeignCurrencyTransactions
Swap contracts
Derivative
Remaining terms	3 years
Coal Contract Derivatives
Derivative
Fair value	0us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_CoalContractMember

X

- Definition

FTP transaction limit

+ References

No definition available.

+ Details

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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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- Definition

The maximum hedge volume for construction material transactions.

+ References

No definition available.

+ Details

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- Definition

The portfolio value at risk limit for foreign currency transactions.

+ References

No definition available.

+ Details

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- Definition

Remaining terms

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- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

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