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Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Natural Gas Swap
Derivative
Fair value	[1]	$ (87)		$ (87)		$ (116)
Natural Gas Total Contracts
Derivative
Fair value		(87)		(87)		(116)
Decrease (increase) in fuel expense		(17)	$ (21)	(45)	$ (32)
Decrease (increase) in purchased power expense		(4)	(5)	(11)	(8)
Fuel Oil and Crude Oil Total Contracts
Derivative
Decrease (increase) in fuel expense		0	$ 0	0	$ 1
Closed derivative contracts
Derivative
Fair value		$ (7)		$ (7)		$ (11)
[1] Fair value amounts presented are based on the net commodity position with the counterparty. Notional amounts disclosed represent the net absolute value of contractual amounts.

X

- 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

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

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=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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