Income Taxes - Reconciliation Between Federal Statutory Income Tax Rate and Effective Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income tax provision at U.S. federal statutory rate	$ 168,545	$ 134,825	$ 116,261
State income taxes, net of U.S. federal benefit	18,733	13,109	14,135
Foreign tax rate differential	5,812	6,675	9,489
Research credits	(6,683)	(5,472)	(3,600)
Valuation allowance	0	(2,183)	(14,451)
Excess tax benefits relating to share-based compensation	(27,638)	(14,907)	(949)
GILTI, FDII, BEAT and FTC	(10,896)	(9,265)	(9,010)
Other	2,776	6,432	12,374
Total provision	$ 150,649	$ 129,214	$ 124,249