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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details) $ in Millions | Mar. 31, 2020 USD ($) megawatts | Sep. 30, 2019 USD ($)
--- | --- | ---
Derivative |  |
Accounts Receivable, before Allowance for Credit Loss, Current | $ 1,182 | $ 1,624
Total derivatives not subject to master netting or similar arrangement | $ 4 | $ 12
Long-term Contract for Purchase of Electric Power [Domain] |  |
Derivative |  |
Megawatts | megawatts | 440 |

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