Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022
Income Tax Contingency [Line Items]
Effective income tax rate	11.20%	10.10%	11.10%	8.30%
Regulatory liabilities	$ 1,355,310		$ 1,355,310		$ 1,358,907
Regulatory excess deferred taxes
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	418,900		418,900
Regulatory liabilities	466,496		466,496		545,021
Regulatory excess deferred taxes | Other Current Liabilities
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	151,400		151,400		$ 159,800
Regulatory excess deferred taxes, to be returned, tranche one
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	332,200		$ 332,200
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	71,600		$ 71,600
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