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Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2015
Natural Gas Swap
Derivative
Fair value	[1]	$ (52)		$ (52)		$ (116)
Natural Gas Total Contracts
Derivative
Fair value		(52)		(52)		(116)
Decrease (increase) in fuel expense		(18)	$ (24)	(62)	$ (56)
Decrease (increase) in purchased power expense		(4)	(6)	(16)	(14)
Fuel Oil and Crude Oil Total Contracts
Derivative
Decrease (increase) in fuel expense		0	$ 0	0	$ 1
Closed derivative contracts
Derivative
Fair value		$ (5)		$ (5)		$ (11)
[1] Fair value amounts presented are based on the net commodity position with the counterparty. Notional amounts disclosed represent the net value of contractual amounts.

X

- 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

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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- Details

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