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Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2012
Natural Gas Futures
Derivative
Notional amount	0		0
Fair value	$ 0		$ 0
Fuel Oil and Crude Oil Futures
Derivative
Notional amount	0		0
Fair value	0		0
Coal Futures
Derivative
Notional amount	0		0
Fair value	0		0
Natural Gas Swap
Derivative
Notional amount	237,485,000		294,462,500
Fair value	(241)		(232)
Fuel Oil and Crude Oil Swap
Derivative
Notional amount	1,349,000		1,390,000
Fair value	1		4
Coal Swap
Derivative
Notional amount	0		0
Fair value	0		0
Natural Gas Option
Derivative
Notional amount	0		0
Fair value	0		0
Fuel Oil and Crude Oil Option
Derivative
Notional amount	0		0
Fair value	0		0
Coal Option
Derivative
Notional amount	0		0
Fair value	0		0
Natural Gas Total Contracts
Derivative
Notional amount	237,485,000		294,462,500
Fair value	(241)		(232)
Decrease (increase) in fuel expense	(28)	0
Decrease (increase) in purchased power expense	(19)	(61)
Fuel Oil and Crude Oil Total Contracts
Derivative
Notional amount	1,349,000		1,390,000
Fair value	1		4
Decrease (increase) in fuel expense	2	5
Coal Contract Total Contracts
Derivative
Notional amount	0		0
Fair value	0		0
Decrease (increase) in fuel expense	(1)	0
Closed derivative contracts
Derivative
Fair value	$ (14)		$ (21)

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_
Data Type:	xbrli:decimalItemType
Balance Type:	na
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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

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

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/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611

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Balance Type:	debit
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