Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Sep. 30, 2024
Regulatory Asset [Line Items]
Regulatory assets	$ 442,252	$ 579,390
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,218,790	1,227,882
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	209,166	257,001
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	635,389	607,032
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	14,643	9,142
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	84,426	73,119
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	230,948	247,250
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	44,218	34,338
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	820	11,243
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	252,035	246,734
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	6,343	10,373
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	36,623	159,762
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	50,154	51,380
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	2,945	3,070
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	38,021	41,742
System Safety and Integrity Riders
Regulatory Asset [Line Items]
Regulatory assets	43,332	38,632
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 11,979	$ 16,454