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Income Taxes (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2022 | Mar. 31, 2022 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021 | Sep. 30, 2021
Income Tax Contingency [Line Items] |  |  |  |  |  |
Effective income tax rate | 9.30% |  | 13.40% | 8.50% | 18.80% |
Regulatory liabilities | $ 1,310,804 |  |  | $ 1,310,804 |  | $ 1,424,195
Winter Storm Uri |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Deferred tax liabilities |  |  |  |  |  | 471,000
Deferred tax asset, decrease during period |  | $ 451,100 |  |  |  |
Domestic Tax Authority |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Operating loss carryforwards | 403,800 |  |  | 403,800 |  | 850,200
State and Local Jurisdiction |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Operating loss carryforwards | 28,100 |  |  | 28,100 |  |
Operating loss carryforwards, federal tax effects | 7,300 |  |  | 7,300 |  |
Tax credit carryforward, amount | 1,700 |  |  | 1,700 |  |
Tax credit carryforward, federal tax impacts | 500 |  |  | $ 500 |  |
Minimum |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Return basis, term |  |  |  | 3 years |  |
Maximum |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Return basis, term |  |  |  | 5 years |  |
Regulatory excess deferred taxes |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Deferred tax liabilities, net | 537,400 |  |  | $ 537,400 |  |
Regulatory liabilities | 586,149 |  |  | 586,149 |  | 705,084
Regulatory excess deferred taxes | Other Current Liabilities |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Deferred tax liabilities, net | 160,200 |  |  | 160,200 |  | $ 155,900
Regulatory excess deferred taxes, to be returned, tranche one |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Deferred tax liabilities, net | 444,200 |  |  | $ 444,200 |  |
Regulatory excess deferred taxes, to be returned, tranche one | Minimum |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Return basis, term |  |  |  | 35 months |  |
Regulatory excess deferred taxes, to be returned, tranche one | Maximum |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Return basis, term |  |  |  | 60 months |  |
Regulatory excess deferred taxes, to be returned, tranche two |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Regulatory liabilities | 78,100 |  |  | $ 78,100 |  |
Regulatory excess deferred taxes, to be returned, tranche two | Minimum |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Return basis, term |  |  |  | 15 years |  |
Regulatory excess deferred taxes, to be returned, tranche two | Maximum |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Return basis, term |  |  |  | 69 years |  |
Regulatory excess deferred taxes, to be returned, tranche three |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Deferred tax liabilities, net | $ 15,100 |  |  | $ 15,100 |  |

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