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Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Sep. 30, 2014
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Regulatory Assets and Liabilities |  |
Current regulatory assets | $ 629 us-gaap_RegulatoryAssetsCurrent | $ 481 us-gaap_RegulatoryAssetsCurrent
Non-current regulatory assets | 9,113 us-gaap_RegulatoryAssetsNoncurrent | 8,994 us-gaap_RegulatoryAssetsNoncurrent
Regulatory assets | 9,742 us-gaap_RegulatoryAssets | 9,475 us-gaap_RegulatoryAssets
Current regulatory liabilities | 174 us-gaap_RegulatoryLiabilityCurrent | 184 us-gaap_RegulatoryLiabilityCurrent
Non-current regulatory liabilities | 1 us-gaap_RegulatoryLiabilityNoncurrent | 0 us-gaap_RegulatoryLiabilityNoncurrent
Regulatory liabilities | 175 us-gaap_RegulatoryLiabilities | 184 us-gaap_RegulatoryLiabilities
Fuel cost adjustment tax equivalents |  |
Regulatory Assets and Liabilities |  |
Current regulatory liabilities | 170 us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_DeferredIncomeTaxChargesMember | 182 us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_DeferredIncomeTaxChargesMember
Unrealized gains/losses on commodity derivatives |  |
Regulatory Assets and Liabilities |  |
Current regulatory liabilities | 4 us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_DeferredDerivativeGainLossMember | 2 us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_DeferredDerivativeGainLossMember
Non-current regulatory liabilities | 1 us-gaap_RegulatoryLiabilityNoncurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_DeferredDerivativeGainLossMember | 0 us-gaap_RegulatoryLiabilityNoncurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_DeferredDerivativeGainLossMember
Deferred nuclear generating units |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 237 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredProjectCostsMember | 237 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredProjectCostsMember
Non-current regulatory assets | 1,153 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredProjectCostsMember | 1,255 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredProjectCostsMember
Unrealized gains/losses on commodity derivatives |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 213 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredDerivativeGainLossMember | 134 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredDerivativeGainLossMember
Non-current regulatory assets | 109 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredDerivativeGainLossMember | 72 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredDerivativeGainLossMember
Environmental agreements |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 63 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalIssueMember | 54 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalIssueMember
Non-current regulatory assets | 79 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalIssueMember | 108 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalIssueMember
Environmental cleanup costs - Kingston ash spill |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 45 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalRestorationCostsMember | 47 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalRestorationCostsMember
Non-current regulatory assets | 365 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalRestorationCostsMember | 421 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalRestorationCostsMember
Fuel cost adjustment receivable/liability |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 70 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredFuelCostsMember | 9 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = us-gaap_DeferredFuelCostsMember
Other current regulatory assets |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 1 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = tve_OthercurrentregulatoryassetsMember | 0 us-gaap_RegulatoryAssetsCurrent / us-gaap_RegulatoryAssetAxis = tve_OthercurrentregulatoryassetsMember
Deferred pension costs and other post-retirement benefits costs |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 4,158 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_PensionCostsMember | 4,297 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = us-gaap_PensionCostsMember
Deferred pension costs due to actions of regulator |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 114 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_DefinedBenefitPlanOtherCostsMember | 0 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_DefinedBenefitPlanOtherCostsMember
Unrealized losses on interest rate derivatives |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 1,306 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_UnrealizedLossesOnInterestRateDerivativesMember | 957 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_UnrealizedLossesOnInterestRateDerivativesMember
Nuclear decommissioning costs |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 882 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_NuclearDesommissioningCostsMember | 931 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_NuclearDesommissioningCostsMember
Non-nuclear decommissioning costs |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 654 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_NonNuclearDecommissioningMember | 645 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_NonNuclearDecommissioningMember
Other non-current regulatory assets |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | $ 293 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_OtherNonCurrentRegulatoryAssetsMember | $ 308 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RegulatoryAssetAxis = tve_OtherNonCurrentRegulatoryAssetsMember

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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