Unaudited Financial Information (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Regulatory Asset [Line Items]
Regulatory assets	$ 2,486,682		$ 414,641
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,447,349		1,367,013
Effective income tax rate	19.80%	22.10%
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 702,393		718,651
Regulatory excess deferred taxes | Other Current Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 62,900		20,900
Regulatory excess deferred taxes | Other Current Liabilities | Minimum
Regulatory Liabilities [Line Items]
Return basis, term	15 years
Regulatory excess deferred taxes | Other Current Liabilities | Maximum
Regulatory Liabilities [Line Items]
Return basis, term	69 years
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 531,625		531,096
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	122,864		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	44,291		57,379
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	25,828		19,554
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	143,685		149,089
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	189,154		183,943
Winter Storm Uri incremental costs
Regulatory Asset [Line Items]
Regulatory assets	2,093,534		0
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	19,636		40,593
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	4,165		4,894
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	30,415		29,839
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 6,093		$ 6,283