Unaudited Financial Information - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Regulatory Asset [Line Items]
Regulatory assets	$ 2,475,545	$ 2,521,441
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,389,519	1,424,195
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	626,874	705,084
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	552,100	541,511
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	83,834	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	27,397	31,110
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	53,370	56,201
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	27,571	19,363
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	37,927	45,922
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	204,915	222,795
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	2,106,521	2,100,728
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	21,366	66,395
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	48,352	45,370
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,521	3,789
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	34,149	32,099
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 18,794	$ 4,343