Income Taxes - Reconciliation of Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
U.S. federal income taxes at statutory rate	21.00%	24.50%	35.00%
State income taxes, net of federal benefit	(3.30%)	5.20%	3.90%
Share-based payments	(4.70%)	(3.90%)	0.00%
Deferred tax asset/liability remeasurement	0.00%	(73.50%)	0.00%
Repatriation transition tax	4.30%	1.80%	0.00%
Non-deductible meals and entertainment	(13.90%)	2.20%	0.60%
Other	(1.80%)	(1.20%)	(1.20%)
Effective tax rate	1.60%	(44.90%)	38.30%