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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivatives Under Financial Trading Program (Details) - Natural Gas Contract Derivatives - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Derivative
Decrease (increase) in fuel expense	$ 0	$ (6)
Decrease (increase) in purchased power expense	$ 0	$ (2)

X

- Definition

The decrease (increase) in fuel expense relating to the FTP.

+ References

No definition available.

+ Details

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Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The decrease (increase) in purchased power expense related to the FTP.

+ References

No definition available.

+ Details

Name:	tve_DecreaseIncreaseInPurchasedPowerExpense
Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

X

- Details

Name:	tve_DerivativesUnderFinancialTradingProgramAxis=srt_NaturalGasReservesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: