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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) $ in Millions		3 Months Ended
		Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2016 USD ($)
Swap
Derivative
Remaining terms		1
Coal Contract
Derivative
Notional amount		25,000,000		20,000,000
Fair value		$ (104)		$ (127)
Natural Gas Contract Derivatives
Derivative
Notional amount		198,000,000		148,000,000
Fair value		$ 17		$ 2
Closed derivative contracts
Derivative
Fair value		(1)		(5)
Natural Gas Contract Derivatives
Derivative
Fair value		(20)		$ (39)
Decrease (increase) in fuel expense		(11)	$ (29)
Decrease (increase) in purchased power expense		$ (3)	$ (7)
Natural Gas swap
Derivative
Notional amount	[1]	15,400,000		21,052,500
Fair value	[1]	$ (20)		$ (39)
Maximum | Coal Contract
Derivative
Remaining terms		2 years
Maximum | Natural Gas Contract Derivatives
Derivative
Remaining terms		4 years
[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

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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- Definition

Remaining terms

+ References

No definition available.

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X

- 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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- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Details

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- Details

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- Details

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- Details

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