Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Sep. 30, 2021
Income Tax Contingency [Line Items]
Effective income tax rate	9.30%		13.40%	8.50%	18.80%
Regulatory liabilities	$ 1,310,804			$ 1,310,804		$ 1,424,195
Winter Storm Uri
Income Tax Contingency [Line Items]
Deferred tax liabilities						471,000
Deferred tax asset, decrease during period		$ 451,100
Domestic Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	403,800			403,800		850,200
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carryforwards	28,100			28,100
Operating loss carryforwards, federal tax effects	7,300			7,300
Tax credit carryforward, amount	1,700			1,700
Tax credit carryforward, federal tax impacts	500			$ 500
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	537,400			$ 537,400
Regulatory liabilities	586,149			586,149		705,084
Regulatory excess deferred taxes | Other Current Liabilities
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	160,200			160,200		$ 155,900
Regulatory excess deferred taxes, to be returned, tranche one
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	444,200			$ 444,200
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	78,100			$ 78,100
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