Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Sep. 30, 2024
Income Tax Contingency [Line Items]
Effective income tax rate	19.20%	16.80%	19.10%	15.60%
Regulatory liabilities	$ 1,218,790		$ 1,218,790		$ 1,227,882
Regulatory excess deferred taxes
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	159,000		159,000
Regulatory liabilities	209,166		209,166		257,001
Regulatory excess deferred taxes | Other Current Liabilities
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	75,500		75,500		$ 79,700
Regulatory excess deferred taxes, to be returned, tranche one
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	109,800		$ 109,800
Regulatory excess deferred taxes, to be returned, tranche one | Minimum
Income Tax Contingency [Line Items]
Return basis, term			12 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	48,200		$ 48,200
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			68 years
Regulatory excess deferred taxes, to be returned, tranche three
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	$ 1,000		$ 1,000