Unaudited Financial Information - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Regulatory Asset [Line Items]
Regulatory assets	$ 2,527,012	$ 414,641
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,362,984	1,367,013
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	694,018	718,651
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	536,317	531,096
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	55,572	19,985
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	20,348	20,348
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	37,358	57,379
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	19,371	19,554
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	135,411	149,089
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	222,263	183,943
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	2,093,779	0
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	27,593	40,593
Regulatory excess deferred taxes
Regulatory Asset [Line Items]
Regulatory assets	8,838	0
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	3,893	4,894
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	31,283	29,839
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 3,952	$ 6,283