Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Sep. 30, 2023
Regulatory Asset [Line Items]
Regulatory assets	$ 451,524	$ 554,928
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,187,372	1,284,334
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	283,073	384,513
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	605,449	582,867
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	20,188	23,093
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	48,544	49,894
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	199,838	215,913
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	30,280	28,054
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	14,033	20,629
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	209,609	229,996
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	12,549	32,115
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	68,296	148,297
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	50,462	47,549
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,112	3,238
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	42,454	54,008
APT annual System Safety and Integrity Rider
Regulatory Asset [Line Items]
Regulatory assets	35,721	0
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 15,288	$ 19,096