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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.

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- 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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- Definition The decrease (increase) in purchased power expense related to the FTP. + References No definition available. + Details
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