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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, 2019 USD ($) megawatts Customers	Sep. 30, 2018 USD ($)
Derivative
Receivables from power sales | $	$ 1,624	$ 1,570
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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Amount, before allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business, classified as current.

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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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