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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Dec. 31, 2020 USD ($) megawatts | Sep. 30, 2020 USD ($)
Derivative |  |
Accounts Receivable, before Allowance for Credit Loss, Current | $ 1,242 | $ 1,401
Total derivatives not subject to master netting or similar arrangement | $ 25 | $ 49
Two Largest Customer Percentage of Total Operating Revenue | 1600.00% |
Moody's, A1 Rating [Member] |  |
Derivative |  |
Banking Counterparties Credit Rating | A3 |
Natural Gas Banking Counterparties Credit Rating | A2 |
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 |

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