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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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Megawatts of summer net capability

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Number of customers that represent the percent of sales

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Amounts due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer) for goods or services (including trade receivables) that have been delivered or sold in the normal course of business.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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