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Regulation (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Regulatory Assets [Line Items]
Regulatory assets	$ 608,729	$ 579,390
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,269,648	1,227,882
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	190,274	257,001
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	641,019	607,032
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	6,879	9,142
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	99,393	73,119
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	291,351	247,250
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	40,732	34,338
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets	262	11,243
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory assets	314,047	246,734
Winter Storm Uri incremental costs
Regulatory Assets [Line Items]
Regulatory assets	5,841	10,373
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	140,626	159,762
Regulatory excess deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	49,793	51,380
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory assets	2,903	3,070
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory assets	39,035	41,742
System Safety and Integrity Riders
Regulatory Assets [Line Items]
Regulatory assets	43,625	38,632
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 12,597	$ 16,454

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
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-Topic 980
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-Name Accounting Standards Codification
-Section 25
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