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Fair Value Measurements Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $) | 12 Months Ended
--- | ---
Sep. 30, 2014 | Sep. 30, 2013
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Commodity contract derivatives | $ 1,000,000 | $ 3,000,000
Commodity contract derivatives | 97,000,000 | 144,000,000
Liabilities |  |
Number of contracts | 2 | 1
Fair Value Measurements |  |
Amount of gain (loss) recognized in income | 57,000,000 |
Private Partnerships |  |
Fair Value Measurements |  |
Balances at beginning of period | 159,000,000 | 53,000,000
Purchases | 39,000,000 | 101,000,000
Issuances | 0 | 0
Sales | (9,000,000) | (4,000,000)
Settlements | 0 | 0
Net unrealized gains (losses) deferred as regulatory assets and liabilities | 25,000,000 | 9,000,000
Balances at end of period | 214,000,000 | 159,000,000
Commodity Contract Derivatives |  |
Fair Value Measurements |  |
Balances at beginning of period | (140,000,000) | (267,000,000)
Purchases | 0 | 0
Issuances | 0 | 0
Sales | 0 | 0
Settlements | 33,000,000 | 0
Net unrealized gains (losses) deferred as regulatory assets and liabilities | 11,000,000 | 127,000,000
Balances at end of period | (96,000,000) | (140,000,000)
Minimum |  |
Assets |  |
Fair value inputs, counterparty credit risk | 2.00% |
Fair value measurements tons per year | 1,000,000,000 | 900,000,000
Price per ton | 11.24 | 10.25
Liabilities |  |
Fair value measurements tons per year | 1,000,000,000 | 900,000,000
Price per ton | 11.24 | 10.25
Maximum |  |
Assets |  |
Fair value inputs, counterparty credit risk | 5.00% | 21.00%
Fair value measurements tons per year | 1,100,000,000 | 1,000,000,000
Price per ton | 67.07 | 85.25
Liabilities |  |
Fair value measurements tons per year | 1,100,000,000 | 1,000,000,000
Price per ton | 67.07 | 85.25
Fair Value, Inputs, Level 3 |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Commodity contract derivatives | 1,000,000 | 3,000,000
Commodity contract derivatives | $ 97,000,000 | $ 143,000,000

X
- Definition The fair value as of the balance sheet date of firm obligations in physical commodity contracts (for example, agricultural products, metals and energy). + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition This item represents the amount of the total realized and unrealized gains or losses for the period which are deferred as regulatory assets and liabilities; the fair value of which assets or liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3). + References No definition available. + Details
Name: | tve_FairValueAssetsOrLiabilitiesMeasuredOnRecurringBasisGainLossDeferredAsRegulatoryAssetsOrLiabilities
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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- Definition The fair value measurement of tons per year used to determine fair value + References No definition available. + Details
Name: | tve_FairValueMeasurementsTonsPerYear
Namespace Prefix: | tve_
Data Type: | num:weightItemType
Balance Type: | na
Period Type: | instant

X
- Definition Issues that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
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Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Purchases that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
Name: | tve_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOrLiabilityPurchases
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Sales that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
Name: | tve_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOrLiabilitySales
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Settlements that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
Name: | tve_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOrLiabilitySettlements
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition This element represents an asset or liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References No definition available. + Details
Name: | tve_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOrLiabilityValue
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- Definition The number of contracts the range applies to. + References No definition available. + Details
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Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition The price per ton used for the determination of fair value. + References No definition available. + Details
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Namespace Prefix: | tve_
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- Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition Risk that one party to the contract will default expressed as a percentage, used as an input to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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X
- Definition The fair value as of the balance sheet date of firm holdings in physical commodity contracts (for example, agricultural products, metals and energy). Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Details
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