Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Income Tax Contingency [Line Items]
Effective income tax rate	14.70%	11.00%
Regulatory liabilities	$ 1,236,749		$ 1,284,334
Regulatory excess deferred taxes
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	295,300
Regulatory liabilities	343,075		384,513
Regulatory excess deferred taxes | Other Current Liabilities
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	111,100		$ 131,300
Regulatory excess deferred taxes, to be returned, tranche one
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	$ 241,900
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	$ 52,400
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	$ 1,000