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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 | 76,175,000 invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember | [1] |  | 76,175,000 invest_DerivativeNonmonetaryNotionalAmount / tve_DerivativesUnderFinancialTradingProgramAxis = tve_NaturalGasSwapMember | [1] |  | 102,227,500 invest_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 | 0 tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember |  | 0 tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember | 1,000,000 tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember |  | 1,000,000 tve_DecreaseIncreaseInFuelExpense / tve_DerivativesUnderFinancialTradingProgramAxis = tve_FuelOilAndCrudeOilTotalContractsMember |  |
Coal Contract Total Contracts |  |  |  |  |  |  |  |
Derivative |  |  |  |  |  |  |  |
Fair value | 0 us-gaap_DerivativeFairValueOfDerivativeNet / tve_DerivativesUnderFinancialTradingProgramAxis = us-gaap_FutureMember |  |  | 0 us-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.

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- 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 + Details
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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 + Details
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- Details
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