Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Income Tax Contingency [Line Items]
Effective income tax rate	5.90%	23.10%
Regulatory liabilities	$ 1,345,370		$ 1,424,195
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	$ 619,000
Regulatory liabilities	667,882		705,084
Regulatory excess deferred taxes | Other Current Liabilities
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	159,200		$ 155,900
Regulatory excess deferred taxes, to be returned, tranche one
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	$ 497,900
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	$ 106,000
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