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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, 2013	Dec. 31, 2012	Sep. 30, 2013
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
Natural Gas Swap
Derivative
Notional amount	142,767,500		152,922,500
Fair value	(134)		(169)
Fuel Oil and Crude Oil Swap
Derivative
Notional amount	0		1,205,000
Fair value	2		3
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
Natural Gas Total Contracts
Derivative
Notional amount	142,767,500		152,922,500
Fair value	(134)		(169)
Decrease (increase) in fuel expense	(15)	(28)
Decrease (increase) in purchased power expense	(6)	(19)
Fuel Oil and Crude Oil Total Contracts
Derivative
Notional amount	0		1,205,000
Fair value	2		3
Decrease (increase) in fuel expense	1	2
Coal Contract Total Contracts
Derivative
Fair value	0		0
Decrease (increase) in fuel expense	0	(1)
Closed derivative contracts
Derivative
Fair value	$ (3)		$ (8)

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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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
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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
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 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611



Reference 2: 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 3: 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=28364263&loc=d3e13495-108611



Reference 4: 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

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