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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, 2015 USD ($)	Sep. 30, 2014 USD ($) mmBtu
Derivative
FTP transaction limit	$ 130
Natural Gas
Derivative
Decrease (increase) in fuel expense	(79)	$ (34)
Decrease (increase) in purchased power expense	(20)	(11)
Fuel Oil and Crude Oil Total Contracts
Derivative
Decrease (increase) in fuel expense	$ 1	$ 2
Natural Gas Swap
Derivative
Notional amount	51,495,000	102,227,500
Unrealized gains (losses) deferred as regulatory liabilities (assets)	$ (116)	$ (103)
Coal Contract Total Contracts
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)	0
Closed Derivative Contracts
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)	$ (11)	$ (5)
Swap contracts
Derivative
Remaining terms	3 years

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

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

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- Definition

FTP transaction limit

+ References

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- Definition

Remaining terms

+ References

No definition available.

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- 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 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 10



-Subparagraph (a)



-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611



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-Publisher FASB



-Name Accounting Standards Codification



-Topic 825



-SubTopic 10



-Section 50



-Paragraph 15



-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13495-108611

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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.

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