Regulation - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Sep. 30, 2024
Regulatory Asset [Line Items]
Regulatory assets	$ 398,789	$ 579,390
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,275,303	1,227,882
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	229,096	257,001
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	625,899	607,032
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	54,794	9,142
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	78,126	73,119
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	236,382	247,250
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	51,006	34,338
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	4,326	11,243
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	239,997	246,734
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	6,812	10,373
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	3,830	159,762
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	50,526	51,380
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	2,987	3,070
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	39,899	41,742
APT annual System Safety and Integrity Rider
Regulatory Asset [Line Items]
Regulatory assets	$ 34,142	38,632
Regulatory asset, recognition period	12 months
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 16,270	$ 16,454