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Income Taxes - Schedule of Income Tax Reconciliation by Applying Federal Income Tax Rate (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Income Tax Disclosure [Abstract] |  |  |  |
U.S. Income Before Income Taxes |  | $ 694,053 | $ 87,255 | $ 641,399
Income Tax Expense, Computed at U.S. Federal Statutory Rate of 21% |  | 145,751 | 18,324 | 134,694
State Income Taxes |  | 33,637 | 1,892 | 36,331
Amortization of Excess Deferred Federal Income Taxes |  | (4,617) | (5,607) | (6,053)
Plant Flow Through Items |  | (5,070) | (6,135) | (2,856)
Stock Compensation |  | 2,245 | 1,758 | 957
Miscellaneous |  | 3,603 | (490) | 1,460
Total Income Taxes | [1] | $ 175,549 | $ 9,742 | $ 164,533
Federal Statutory Rate |  | 21.00% | 21.00% | 21.00%
[1] | The Company considers this line to be a significant expense.

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