Unaudited Financial Information (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2020	Sep. 30, 2019
Regulatory Asset [Line Items]
Regulatory assets	$ 300,031	$ 284,547
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,405,246	1,386,126
Deferred Income Tax Charge - Remeasurement Due to Law Change under the Reg Liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,100
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	723,400	726,307
Regulatory excess deferred taxes | Other Current Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 20,900	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	$ 2,559	5,238
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	528,932	528,893
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	48,917	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	62,461	78,402
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	21,923	16,120
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	79,108	86,089
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	180,390	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	5,259	6,551
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	28,700	26,418
Rate case costs
Regulatory Asset [Line Items]
Regulatory assets	401	1,346
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 6,173	$ 8,483