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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, 2018 USD ($) Customers megawatts	Sep. 30, 2017 USD ($)
Derivative
Receivables from power sales | $	$ 1,570	$ 1,441
Credit of Customers
Derivative
Number of customers that represent the percent of sales | Customers	2
Percent of total sales by customers	17.00%
Power Purchase Agreement
Derivative
Megawatts | megawatts	440

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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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