Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Regulatory Assets [Line Items]
Regulatory assets	$ 2,521,441	$ 414,641
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 1,424,195	$ 1,367,013
Effective income tax rate	18.80%	19.50%
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 705,084	$ 718,651
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	541,511	531,096
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	52,553	19,985
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	18,373	20,348
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory Liabilities	31,110	57,379
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	56,201	0
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	19,363	19,554
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	155,900	20,900
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets	45,922	149,089
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory assets	222,795	183,943
Winter Storm Uri incremental costs
Regulatory Assets [Line Items]
Regulatory assets	2,100,728	0
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	66,395	40,593
Regulatory excess deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	45,370	0
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory assets	3,789	4,894
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory assets	32,099	29,839
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 4,343	$ 6,283