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Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) (USD $)	3 Months Ended			6 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Mar. 31, 2015		Mar. 31, 2014	Sep. 30, 2014
Natural Gas Swap
Derivative
Notional amount	76,175,000invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember	[1]		76,175,000invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember	[1]		102,227,500invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember	[1]
Fair value	$ (157,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember	[1]		$ (157,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember	[1]		$ (103,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember	[1]
Natural Gas Total Contracts
Derivative
Fair value	(157,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember			(157,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember			(103,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember	[1]
Decrease (increase) in fuel expense	(21,000,000)tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember		(3,000,000)tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember	(32,000,000)tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember		(18,000,000)tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember
Decrease (increase) in purchased power expense	(5,000,000)tve_DecreaseIncreaseInPurchasedPowerExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember		(1,000,000)tve_DecreaseIncreaseInPurchasedPowerExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember	(8,000,000)tve_DecreaseIncreaseInPurchasedPowerExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember		(7,000,000)tve_DecreaseIncreaseInPurchasedPowerExpense / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_NaturalGasReservesMember
Fuel Oil and Crude Oil Total Contracts
Derivative
Decrease (increase) in fuel expense	0tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember		0tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember	1,000,000tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember		1,000,000tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember
Coal Contract Total Contracts
Derivative
Fair value	0us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_FutureMember			0us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_FutureMember
Closed derivative contracts
Derivative
Fair value	$ (10,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeByNatureAxis = tve_ClosedDerivativeContractsMember			$ (10,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeByNatureAxis = tve_ClosedDerivativeContractsMember			$ (5,000,000)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeByNatureAxis = tve_ClosedDerivativeContractsMember
[1] Fair value amounts presented are based on the net commodity position with the counterparty. Notional amounts disclosed represent the net absolute value of contractual amounts.

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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X

- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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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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Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
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X

- 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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- Details

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- Details

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