Unaudited Financial Information - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Sep. 30, 2022
Regulatory Asset [Line Items]
Regulatory assets	$ 2,471,810	$ 2,605,372
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,339,967	1,358,907
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	506,406	545,021
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	572,384	568,307
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	39,013	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	37,443	31,138
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	151,843	156,857
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	27,141	23,013
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	26,977	31,122
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	164,818	235,972
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	2,125,787	2,109,454
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	41,096	119,742
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	48,033	47,311
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,364	3,406
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	38,647	36,898
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 23,088	$ 21,467