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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivatives Under Financial Trading Program (Details) $ in Millions	12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Derivative
FTP transaction limit	$ 130
Natural Gas Swap
Derivative
Notional amount	0	2,800,000
Unrealized gains (losses) deferred as regulatory liabilities (assets)	$ 0	$ (5)
Natural Gas Contract Derivatives
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)	0	(5)
Decrease (increase) in fuel expense	(6)	(29)
Decrease (increase) in purchased power expense	$ (2)	(7)
Closed Derivative Contracts
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)		$ (3)

X

- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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Namespace Prefix:	invest_
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Balance Type:	na
Period Type:	instant

X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
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X

- Definition

FTP transaction limit

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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X

- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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- 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

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- Details

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- Details

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- Details

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