Provision for Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Income Tax Contingency [Line Items]
Deferred tax assets, gross	$ 3,574,000	$ 3,840,000
Fuel tax credits	300,000	300,000
Goodwill expected to be deductible to tax purposes	19,000,000.0	14,900,000
Income tax benefit related to tax act	4,600,000
Permanent tax benefit		(2,300,000)
Amount resulting from deduction fair market value of options over exercise price	1,400,000	1,400,000
Unrecognized tax benefits	0	0
State
Income Tax Contingency [Line Items]
Operating loss carryforwards	$ 31,600,000	$ 38,300,000