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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, 2020 USD ($) megawatts Customers | Sep. 30, 2019 USD ($)
Derivative |  |
Receivables from power sales | $ | $ 1,401 | $ 1,624
Credit of Customers |  |
Derivative |  |
Number of customers that represent the percent of sales | Customers | 2 |
Percent of total sales by customers | 17.00% |
Long-term Contract for Purchase of Electric Power [Domain] |  |
Derivative |  |
Megawatts | megawatts | 440 |

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