Unaudited Financial Information - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Sep. 30, 2021
Regulatory Asset [Line Items]
Regulatory assets	$ 2,513,357	$ 2,521,441
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,310,804	1,424,195
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	586,149	705,084
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	558,542	541,511
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	45,874	52,553
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	18,373	18,373
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	26,689	31,110
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	51,954	56,201
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	23,223	19,363
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	34,482	45,922
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	225,279	222,795
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	2,107,820	2,100,728
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	42,818	66,395
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	48,746	45,370
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,448	3,789
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	35,212	32,099
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 15,552	$ 4,343