Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Income Tax Contingency [Line Items]
Deferred tax liabilities, other	$ 47,086	$ 22,265
Charitable contribution carryforwards	6,000
Remeasured federal effects	500
Unrecognized tax benefits, period increase	32,800
Interest and penalties	1,400	700	$ 2,200
Payment of interest and penalties accrued	10,400	8,200	7,900
Regulatory liability	1,424,195	1,367,013
Income tax benefit	$ 0	$ 20,962	$ 0
Effective income tax rate	18.80%	19.50%
Deferred tax liability, regulated operations	$ 532,300
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	$ 115,300
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	$ 12,100
Regulatory excess deferred taxes
Income Tax Contingency [Line Items]
Regulatory liability	705,084	$ 718,651
Regulatory excess deferred taxes | Other Current Liabilities
Income Tax Contingency [Line Items]
Regulatory liability	155,900	$ 20,900
Winter Storm Uri
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	471,000
Deferred tax liabilities, other	471,000
Valuation Allowance, Tax Credit Carryforward
Income Tax Contingency [Line Items]
Valuation allowance	700
Domestic Tax Authority
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	850,200
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	61,200
Federal alternative minimum tax credit carryforwards	1,700
Federal effects	$ 16,200