Income Taxes - Reconciliation of Statutory Federal Income Tax Rate (Detail)		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
U.S. federal income taxes at statutory rate	35.00%	21.00%	21.00%	24.50%
State income taxes, net of federal benefit		4.10%	(3.30%)	5.20%
Share-based payments		(0.60%)	(4.70%)	(3.90%)
Deferred tax asset/liability remeasurement		0.60%	0.00%	(73.50%)
Repatriation transition tax		0.00%	4.30%	1.80%
Non-deductible meals and entertainment		(0.90%)	(13.90%)	2.20%
Other		0.70%	(1.80%)	(1.20%)
Effective tax rate		24.90%	1.60%	(44.90%)