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Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014
Natural Gas Swap
Derivative
Notional amount	88,195,000invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember		102,227,500invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember
Fair value	$ (159)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember		$ (103)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember
Natural Gas Total Contracts
Derivative
Notional amount	88,195,000invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember		102,227,500invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember
Fair value	(159)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember		(103)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember
Decrease (increase) in fuel expense	(12)tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember	(15)tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember
Decrease (increase) in purchased power expense	(3)tve_DecreaseIncreaseInPurchasedPowerExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember	(6)tve_DecreaseIncreaseInPurchasedPowerExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember
Fuel Oil and Crude Oil Total Contracts
Derivative
Decrease (increase) in fuel expense	1tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember	1tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember
Coal Contract Total Contracts
Derivative
Fair value	0us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_CoalContractMember
Closed derivative contracts
Derivative
Fair value	$ (8)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeByNatureAxis = tve_ClosedDerivativeContractsMember		$ (5)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeByNatureAxis = tve_ClosedDerivativeContractsMember

X

- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

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- Definition

The decrease (increase) in fuel expense relating to the FTP.

+ References

No definition available.

+ Details

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- Definition

The decrease (increase) in purchased power expense related to the FTP.

+ References

No definition available.

+ Details

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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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- Details

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