Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 1,367,013	$ 1,386,126
Regulatory Assets [Line Items]
Regulatory Assets	414,641	284,547
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	718,651	726,307
Regulatory Cost of Service Reserve
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,716	5,238
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	531,096	528,893
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	19,985	14,112
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	20,348	17,054
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory Liabilities	57,379	78,402
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	17,838	16,120
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	20,900	21,200
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets	149,089	86,089
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory Assets	183,943	131,894
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory Assets	40,593	23,766
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory Assets	4,894	6,551
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory Assets	29,839	26,418
Other
Regulatory Assets [Line Items]
Regulatory Assets	6,283	$ 9,829
State and Local Jurisdiction | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 12,100