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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Counterparty Credit Risk (Details) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2016 USD ($) Customers merchants megawatts | Sep. 30, 2015 USD ($) Customers
Derivative |  |  |
Receivables from power sales |  | $ 1,637 | $ 1,509
Coal counterparties in asset position | [1] | $ 9 | $ 26
Number of active future commission merchants | merchants |  | 2 |
Credit of Customers |  |  |
Derivative |  |  |
Number of customers that represent the percent of sales | Customers |  | 6 | 6
Percent of total sales by customers |  | 33.00% | 33.00%
Power Purchase Agreement |  |  |
Derivative |  |  |
Megawatts | megawatts |  | 440 |
Coal Contract Derivatives |  |  |
Derivative |  |  |
Coal counterparties in asset position |  | $ 4 |
[1] | There are no derivative contracts subject to a master netting arrangement or similar agreement which are not offset in the balance sheets.

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