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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, 2016 USD ($)	Sep. 30, 2015 USD ($)
Derivative
FTP transaction limit	$ 130
Natural Gas Swap
Derivative
Notional amount	21,052,500	51,495,000
Unrealized gains (losses) deferred as regulatory liabilities (assets)	$ (39)	$ (116)
Natural Gas Contract Derivatives
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)	(39)	(116)
Decrease (increase) in fuel expense	(75)	(79)
Decrease (increase) in purchased power expense	(19)	(20)
Fuel oil/crude oil
Derivative
Decrease (increase) in fuel expense	0	1
Closed Derivative Contracts
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)	$ (5)	$ (11)
Swap contracts
Derivative
Remaining terms	2

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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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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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=73719730&loc=d3e13433-108611

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=73719730&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.

+ References

No definition available.

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