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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, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015 | Sep. 30, 2015
Natural Gas Swap |  |  |  |  |  |
Derivative |  |  |  |  |  |
Fair value | [1] | $ (52) |  | $ (52) |  | $ (116)
Natural Gas Total Contracts |  |  |  |  |  |
Derivative |  |  |  |  |  |
Fair value |  | (52) |  | (52) |  | (116)
Decrease (increase) in fuel expense |  | (18) | $ (24) | (62) | $ (56) |
Decrease (increase) in purchased power expense |  | (4) | (6) | (16) | (14) |
Fuel Oil and Crude Oil Total Contracts |  |  |  |  |  |
Derivative |  |  |  |  |  |
Decrease (increase) in fuel expense |  | 0 | $ 0 | 0 | $ 1 |
Closed derivative contracts |  |  |  |  |  |
Derivative |  |  |  |  |  |
Fair value |  | $ (5) |  | $ (5) |  | $ (11)
[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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