Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Income Tax Contingency [Line Items]
Charitable contribution carryforwards	$ 8,200
Remeasured federal effects	500
Unrecognized tax benefits, period increase	58,600	$ 52,700
Unrecognized tax benefits, deferred tax liabilities	12,000	12,800
Interest and penalties	3,400	1,300	$ 1,400
Payment of interest and penalties accrued	15,100	11,700	$ 10,400
Regulatory liabilities	1,284,334	1,353,170
Deferred tax liabilities, net	2,304,974	1,999,505
Regulatory excess deferred taxes
Income Tax Contingency [Line Items]
Regulatory liabilities	384,513	545,021
Regulatory excess deferred taxes to be returned tranche one
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	$ 279,400
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	$ 53,500
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	$ 4,000
Other current liabilities | Regulatory excess deferred taxes
Income Tax Contingency [Line Items]
Regulatory liabilities	131,300	$ 159,800
Domestic Tax Authority
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	458,700
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	45,400
Federal effects	11,900
Federal alternative minimum tax credit carryforwards	$ 1,900