Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Sep. 30, 2023
Regulatory Assets [Line Items]
Regulatory assets	$ 579,390	$ 554,928
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,227,882	1,284,334
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	257,001	384,513
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	607,032	582,867
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	9,142	23,093
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	73,119	49,894
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	247,250	215,913
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	34,338	28,054
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets	11,243	20,629
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory assets	246,734	229,996
Winter Storm Uri incremental costs
Regulatory Assets [Line Items]
Regulatory assets	10,373	32,115
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	159,762	148,297
Regulatory excess deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	51,380	47,549
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory assets	3,070	3,238
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory assets	41,742	54,008
APT annual System Safety and Integrity Rider
Regulatory Assets [Line Items]
Regulatory assets	38,632	0
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 16,454	$ 19,096