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Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014 | Sep. 30, 2014
Natural Gas Swap |  |  |  |  |  |
Derivative |  |  |  |  |  |
Fair value | [1] | $ (125) |  | $ (125) |  | $ (103)
Natural Gas Total Contracts |  |  |  |  |  |
Derivative |  |  |  |  |  |
Fair value |  | (125) |  | (125) |  | (103)
Decrease (increase) in fuel expense |  | (24) | $ (5) | (56) | $ (23) |
Decrease (increase) in purchased power expense |  | (6) | (1) | (14) | (9) |
Fuel Oil and Crude Oil Total Contracts |  |  |  |  |  |
Derivative |  |  |  |  |  |
Decrease (increase) in fuel expense |  | 0 | $ 0 | 1 | $ 2 |
Coal Contract Total Contracts |  |  |  |  |  |
Derivative |  |  |  |  |  |
Fair value |  | 0 |  | 0 |  |
Closed derivative contracts |  |  |  |  |  |
Derivative |  |  |  |  |  |
Fair value |  | $ (10) |  | $ (10) |  | $ (5)
[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.

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