Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Sep. 30, 2024
Regulatory Asset [Line Items]
Regulatory assets	$ 522,516	$ 579,390
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,223,408	1,227,882
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	249,790	257,001
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	615,371	607,032
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	3,731	9,142
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	76,418	73,119
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	241,816	247,250
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	36,282	34,338
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	7,885	11,243
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	189,939	246,734
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	8,834	10,373
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	169,203	159,762
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	50,941	51,380
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,028	3,070
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	41,245	41,742
APT annual System Safety and Integrity Rider
Regulatory Asset [Line Items]
Regulatory assets	$ 35,290	38,632
Regulatory asset, recognition period	12 months
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 16,151	$ 16,454