Regulation (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Sep. 30, 2025
Regulatory Asset [Line Items]
Regulatory assets	$ 597,580	$ 608,729
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,261,109	1,269,648
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	171,412	190,274
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	638,265	641,019
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	4,737	6,879
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	114,110	99,393
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	289,695	291,351
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	42,890	40,732
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	234	262
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	299,641	314,047
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	4,069	5,841
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	151,187	140,626
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	49,468	49,793
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	2,861	2,903
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	36,972	39,035
System Safety and Integrity Riders
Regulatory Asset [Line Items]
Regulatory assets	39,955	43,625
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 13,193	$ 12,597