Income Taxes - Reconciliation Between Federal Statutory Income Tax Rate and Effective Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	24.50%	35.00%	35.00%
Income tax provision at U.S. federal statutory rate	$ 46,119	$ 52,963	$ 50,599
State income taxes, net of U.S. federal benefit	3,429	2,193	2,244
Foreign tax rate differential	(42)	(1,761)	(4,661)
Intercompany interest	0	(477)	(1,223)
Research credits	(3,486)	(2,572)	(4,398)
Domestic production deduction	(3,041)	(3,075)	(3,726)
Amended returns/audit settlements/statute expirations	(2,349)	(1,296)	(248)
Foreign	4,040	744	(1,702)
Valuation allowance	1,907	2,512	1,262
Foreign tax credit	1,320	(1,342)	(3,286)
Excess tax benefits relating to stock-based compensation	(22,253)	(24,746)	0
Tax effect of the Tax Act	23,579	0	0
Other	(3,628)	(75)	260
Total provision	$ 45,595	$ 23,068	$ 35,121