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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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Megawatts of summer net capability

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Number of active future commission merchants to clear commodity contracts.

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Number of customers that represent the percent of sales

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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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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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