Unaudited Financial Information - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Regulatory Asset [Line Items]
Regulatory assets	$ 513,931	$ 2,605,372
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,355,310	1,358,907
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	466,496	545,021
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	568,778	568,307
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	97,407	28,834
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	5,737	5,737
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	39,360	31,138
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	146,829	156,857
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	30,703	23,013
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	24,096	31,122
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	203,010	235,972
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	121,493	2,109,454
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	49,156	119,742
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	47,656	47,311
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,322	3,406
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	51,967	36,898
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 13,231	$ 21,467