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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Derivative
Forward Contract Derivative Asset, at Fair Value		$ 34		$ 34		$ 19
Coal Contract
Derivative
Notional amount		20,000,000		20,000,000		17,000,000
Fair value		$ (52)		$ (52)		$ (67)
Natural Gas Contract Derivatives
Derivative
Notional amount		299,000,000		299,000,000		271,000,000
Fair value		$ (13)		$ (13)		$ 7
Natural Gas Contract Derivatives
Derivative
Fair value		0		0		$ (5)
Decrease (increase) in fuel expense		0	$ (5)	(6)	$ (17)
Decrease (increase) in purchased power expense		$ 0	$ (1)	$ (2)	$ (4)
Natural Gas swap
Derivative
Notional amount	[1]	0		0		2,800,000
Fair value	[1]	$ 0		$ 0		$ (5)
Fair Value, Inputs, Level 2
Derivative
Forward Contract Derivative Asset, at Fair Value		$ 34		$ 34		$ 19
[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

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

No definition available.

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- Definition

Fair value as of the balance sheet date of forward contract derivative assets.

+ 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=77997519&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=77997519&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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