Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Sep. 30, 2021
Regulatory Assets [Line Items]
Regulatory assets	$ 2,605,372	$ 2,521,441
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,358,907	1,424,195
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	545,021	705,084
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	568,307	541,511
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	28,834	52,553
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	5,737	18,373
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory Liabilities	31,138	31,110
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	156,857	56,201
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	23,013	19,363
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets	31,122	45,922
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory assets	235,972	222,795
Winter Storm Uri incremental costs
Regulatory Assets [Line Items]
Regulatory assets	2,109,454	2,100,728
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	119,742	66,395
Regulatory excess deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	47,311	45,370
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory assets	3,406	3,789
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory assets	36,898	32,099
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 21,467	$ 4,343