Unaudited Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Sep. 30, 2020
Regulatory Asset [Line Items]
Regulatory assets	$ 353,246	$ 414,641
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,351,370	1,367,013
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	713,993	718,651
Regulatory excess deferred taxes | Other Current Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 18,800	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	$ 527,087	531,096
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	15,196	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	55,313	57,379
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	19,433	19,554
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	146,734	149,089
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	155,526	183,943
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	11,322	40,593
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	4,529	4,894
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	30,166	29,839
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 4,969	$ 6,283