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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2021 USD ($) megawatts | Jun. 30, 2021 USD ($) megawatts | Sep. 30, 2020 USD ($)
Derivative |  |  |
Accounts Receivable, before Allowance for Credit Loss, Current | $ 1,373 | $ 1,373 | $ 1,401
Total derivatives not subject to master netting or similar arrangement | $ 110 | $ 110 | $ 49
Two Largest Customer Percentage of Total Operating Revenue | 1800.00% |  |
Moody's, A1 Rating [Member] |  |  |
Derivative |  |  |
Banking Counterparties Credit Rating |  | A2 |
Natural Gas Banking Counterparties Credit Rating |  | Aa2 |
Moody's, Caa1 Rating [Member] |  |  |
Derivative |  |  |
Natural Gas Banking Counterparties Credit Rating |  | B1 |
Long-term Contract for Purchase of Electric Power [Domain] |  |  |
Derivative |  |  |
Megawatts | megawatts | 440 | 440 |

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