Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2021
Income Tax Contingency [Line Items]
Effective income tax rate	10.10%	17.20%	8.30%	19.80%
Regulatory liabilities	$ 1,389,519		$ 1,389,519		$ 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	438,000		438,000		850,200
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carryforwards	31,100		31,100
Operating loss carryforwards, federal tax effects	8,200		8,200
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	578,500		$ 578,500
Regulatory liabilities	626,874		626,874		705,084
Regulatory excess deferred taxes | Other Current Liabilities
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	156,000		156,000		$ 155,900
Regulatory excess deferred taxes, to be returned, tranche one
Income Tax Contingency [Line Items]
Deferred tax liabilities, net	458,300		$ 458,300
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	105,100		$ 105,100
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