Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Regulatory Asset [Line Items]
Regulatory assets	$ 405,368	$ 554,928
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,248,781	1,284,334
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	315,020	384,513
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	594,815	582,867
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	57,680	23,093
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	40,233	49,894
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	205,718	215,913
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	35,315	28,054
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	16,174	20,629
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	210,795	229,996
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	15,510	32,115
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	25,928	148,297
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	51,654	47,549
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,154	3,238
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	66,535	54,008
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 15,618	$ 19,096