Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 1,386,126	$ 1,510,997
Regulatory Cost of Service Liability	5,200	22,500
Regulatory Assets [Line Items]
Regulatory Assets	284,547	143,811
Deferred tax liability, regulated operations	726,300	744,900
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	726,307	744,895
Regulatory Cost of Service Reserve
Regulatory Liabilities [Line Items]
Regulatory Liabilities	5,238	22,508
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	528,893	522,175
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	14,112	94,705
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	17,054	12,887
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory Liabilities	78,402	35,228
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	69,113
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	16,120	9,486
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets	86,089	6,496
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory Assets	131,894	96,739
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory Assets	23,766	1,927
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory Assets	6,551	8,702
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory Assets	26,418	20,467
Rate case costs
Regulatory Assets [Line Items]
Regulatory Assets	1,346	2,741
Other
Regulatory Assets [Line Items]
Regulatory Assets	8,483	6,739
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 21,200	$ 5,200