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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details) $ in Millions | 9 Months Ended |
--- | --- | ---
Jun. 30, 2016 USD ($) merchants Customers megawatts | Jun. 30, 2015 Customers | Sep. 30, 2015 USD ($)
Derivative |  |  |  |
Number of active future commission merchants | merchants |  | 2 |  |
Derivative Asset | [1] | $ 12 |  | $ 26
Credit of Customers |  |  |  |
Derivative |  |  |  |
Number of customers that represent the percent of sales | Customers |  | 6 | 6 |
Sales Concentration |  | 33.00% |  |
Megawatts of Power Purchase Agreement |  |  |  |
Derivative |  |  |  |
Megawatts of Power Purchase Agreement | megawatts |  | 440 |  |
Maximum | Coal Contract |  |  |  |
Derivative |  |  |  |
Derivative Asset |  | $ 1 |  |
[1] | There are no derivative contracts subject to a master netting arrangement or similar agreement that are not offset in the consolidated balance sheets.

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