Income Taxes - Reconciliation of Income Tax Provision at Federal Statutory Income Tax Rate and Effective Tax Rate (Detail)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	24.50%	35.00%
State taxes, net of federal benefit	1.80%	2.60%	2.70%
Federal research and development tax credit	(12.80%)	(5.20%)	(7.40%)
Remeasurement of net deferred tax assets		4.20%
Share-based compensation	(6.70%)	(2.80%)	4.50%
Foreign-derived intangible income	(3.20%)
Other	(1.90%)	(0.60%)	(0.50%)
Effective income tax rate	(1.80%)	22.70%	34.30%