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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)
Derivative
Forward Contract Derivative Asset, at Fair Value	$ 59		$ 59		$ 45
Coal Contract
Derivative
Notional amount	15,000,000		15,000,000		20,000,000
Fair value	$ 38		$ 38		$ 58
Natural Gas Contract Derivatives
Derivative
Notional amount	360,000,000		360,000,000		359,000,000
Fair value	$ (10)		$ (10)		$ 2
Natural Gas Contract Derivatives
Derivative
Decrease (increase) in fuel expense	0	$ 0	0	$ (6)
Decrease (increase) in purchased power expense	0	$ 0	0	$ (2)
Fair Value, Inputs, Level 2
Derivative
Forward Contract Derivative Asset, at Fair Value	$ 59		$ 59		$ 45

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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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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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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