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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details)	Mar. 31, 2015 Customers	Sep. 30, 2014 Customers
Derivative
Number of active future commission merchants	2tve_NumberOfActiveFutureCommissionMerchants
Megawatts	700tve_Megawatts
Credit of Customers [Member]
Derivative
Number of customers that represent the concentration of accounts receivable percent outstanding	3tve_NumberOfCustomersThatRepresentPercentOfOutstandingAccountsReceivable / us-gaap_DerivativeByNatureAxis = tve_CreditOfCustomersMember	3tve_NumberOfCustomersThatRepresentPercentOfOutstandingAccountsReceivable / us-gaap_DerivativeByNatureAxis = tve_CreditOfCustomersMember
Percent of total outstanding accounts receivables	24.00%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

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

X

- Details

Name:	us-gaap_DerivativeByNatureAxis=tve_CreditOfCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DerivativeByNatureAxis=us-gaap_LongTermContractForPurchaseOfElectricPowerDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: