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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details)
Dec. 31, 2014
Customers
Sep. 30, 2014
Derivative    
Number of active future commission merchants 2tve_NumberOfActiveFutureCommissionMerchants  
Credit of Customers [Member]    
Derivative    
Number of customers that represent the percent of outstanding accounts receivable 3tve_NumberOfCustomersThatRepresentPercentOfOutstandingAccountsReceivable
/ us-gaap_DerivativeByNatureAxis
= tve_CreditOfCustomersMember
 
Percent of total outstanding accounts receivables 23.00%tve_PercentOfTotalOutstandingAccountsReceivables
/ us-gaap_DerivativeByNatureAxis
= tve_CreditOfCustomersMember
27.00%tve_PercentOfTotalOutstandingAccountsReceivables
/ us-gaap_DerivativeByNatureAxis
= tve_CreditOfCustomersMember
Power Purchase Agreement    
Derivative    
Megawatts 440tve_Megawatts
/ us-gaap_DerivativeByNatureAxis
= us-gaap_LongTermContractForPurchaseOfElectricPowerDomain