Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Regulatory Asset [Line Items]
Regulatory assets	$ 471,795	$ 554,928
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,236,749	1,284,334
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	343,075	384,513
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	584,640	582,867
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	20,889	23,093
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	47,768	49,894
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	211,600	215,913
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	28,777	28,054
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	18,295	20,629
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	167,965	229,996
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	28,328	32,115
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	129,891	148,297
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	47,772	47,549
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,196	3,238
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	59,830	54,008
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 16,518	$ 19,096