Unaudited Financial Information (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2020	Sep. 30, 2019
Regulatory Asset [Line Items]
Regulatory assets	$ 266,508	$ 284,547
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,458,701	1,386,126
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	715,807	726,307
Regulatory excess deferred taxes | Other Current Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 21,400	21,200
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	46 years
Regulatory cost of service reserve
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 3,770	5,238
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	521,319	528,893
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	115,112	14,112
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	17,054	17,054
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	68,048	78,402
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	17,591	16,120
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	80,955	86,089
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	142,075	131,894
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	0	23,766
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	5,652	6,551
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	27,811	26,418
Rate case costs
Regulatory Asset [Line Items]
Regulatory assets	2,250	1,346
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 7,765	$ 8,483