Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Income Tax Contingency [Line Items]
Deferred tax liabilities, other	$ 45,063,000	$ 47,086,000
Charitable contribution carryforwards	0
Remeasured federal effects	500,000
Unrecognized tax benefits, period increase	52,700,000
Unrecognized tax benefits, deferred tax liabilities	12,800,000
Interest and penalties	1,300,000	1,400,000	$ 700,000
Payment of interest and penalties accrued	11,700,000	10,400,000	$ 8,200,000
Regulatory liability	1,358,907,000	1,424,195,000
Deferred tax liability, regulated operations	404,200,000
Regulatory excess deferred taxes
Income Tax Contingency [Line Items]
Regulatory liability	545,021,000	705,084,000
Regulatory excess deferred taxes | Other current liabilities
Income Tax Contingency [Line Items]
Regulatory liability	$ 159,800,000	$ 155,900,000
Regulatory excess deferred taxes to be returned tranche one | Minimum
Income Tax Contingency [Line Items]
Return period	35 months
Regulatory excess deferred taxes to be returned tranche one | Maximum
Income Tax Contingency [Line Items]
Return period	60 months
Regulatory excess deferred taxes to be returned tranche two
Income Tax Contingency [Line Items]
Deferred tax liability, regulated operations	$ 78,400,000
Regulatory excess deferred taxes to be returned tranche two | Minimum
Income Tax Contingency [Line Items]
Return period	15 years
Regulatory excess deferred taxes to be returned tranche two | Maximum
Income Tax Contingency [Line Items]
Return period	69 years
Regulatory excess deferred taxes to be returned tranche three
Income Tax Contingency [Line Items]
Deferred tax liability, regulated operations	$ 15,100,000
Winter Storm Uri
Income Tax Contingency [Line Items]
Deferred tax liabilities, other	471,000,000
Deferred tax asset, decrease during period	450,300,000
Domestic tax authority
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	441,300,000
State and local jurisdiction
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	43,800,000
Federal effects	11,400,000
Federal alternative minimum tax credit carryforwards	$ 1,900,000