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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Counterparty Credit Risk (Details) | Sep. 30, 2014 Customers | Sep. 30, 2013 Customers
--- | --- | ---
Derivative |  |
Number of active future commission merchants | 2 |
Credit of Customers |  |
Derivative |  |
Number of customers that represent the percent of outstanding accounts receivable | 3 | 3
Percent of total outstanding accounts receivables by customers | 27.00% | 27.00%
Power Purchase Agreement |  |
Derivative |  |
Megawatts in a power purchase agreement that expires on March 31, 2032 | 440 |

X
- Definition Megawatts of summer net capability + References No definition available. + Details
Name: | tve_Megawatts
Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of active future commission merchants to clear commodity contracts. + References No definition available. + Details
Name: | tve_NumberOfActiveFutureCommissionMerchants
Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of customers that represent the percent of outstanding accounts receivable. + References No definition available. + Details
Name: | tve_NumberOfCustomersThatRepresentPercentOfOutstandingAccountsReceivable
Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percent of total outstanding accounts receivables represented by a certain number of customers. + References No definition available. + Details
Name: | tve_PercentOfTotalOutstandingAccountsReceivables
Namespace Prefix: | tve_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration