Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Deferred tax assets:
Employee benefit plans	$ 50,576	$ 57,094
Net operating loss carryforwards	504,121	485,061
Charitable and other credit carryforwards	10,084	1,903
Regulatory excess deferred tax	76,943	110,548
Lease asset	58,633	50,007
Other	42,257	44,035
Total deferred tax assets	742,614	748,648
Valuation allowance	(351)	(523)
Net deferred tax assets	742,263	748,125
Deferred tax liabilities:
Difference in net book value and net tax value of assets	(2,674,341)	(2,431,757)
Gas cost adjustments	(47,822)	(43,964)
Winter Storm Uri regulatory asset	(28,116)	(20,710)
Lease liability	(51,666)	(50,007)
Rate deferral adjustment	(47,218)	(49,309)
Interest rate agreements	(149,969)	(106,820)
Other	(48,105)	(45,063)
Total deferred tax liabilities	(3,047,237)	(2,747,630)
Net deferred tax liabilities	$ (2,304,974)	$ (1,999,505)