CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Paid-in- Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance (in shares) at Sep. 30, 2016		30,935
Beginning Balance at Sep. 30, 2016	$ 481,316	$ 309	$ 1,188,913	$ (2,136,760)	$ 1,505,866	$ (77,012)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	61,222		61,222
Issuance of treasury stock under employee stock plans (in shares)		774
Issuance of treasury stock under employee stock plans	(25,758)	$ 8	(54,704)	28,938
Repurchases of common stock (in shares)		(1,466)
Repurchases of common stock	(193,290)	$ (15)		(193,275)
Dividends paid	(1,238)				(1,238)
Net income	133,414				133,414
Foreign currency translation adjustments	10,517					10,517
Ending Balance (in shares) at Sep. 30, 2017		30,243
Ending Balance at Sep. 30, 2017	466,183	$ 302	1,195,431	(2,301,097)	1,638,042	(66,495)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	74,814		74,814
Issuance of treasury stock under employee stock plans (in shares)		633
Issuance of treasury stock under employee stock plans	(33,181)	$ 7	(59,194)	26,006
Repurchases of common stock (in shares)		(1,861)
Repurchases of common stock	(336,935)	$ (19)		(336,916)
Net income	126,482				126,482
Foreign currency translation adjustments	(9,926)					(9,926)
Ending Balance (in shares) at Sep. 30, 2018		29,015
Ending Balance at Sep. 30, 2018	287,437	$ 290	1,211,051	(2,612,007)	1,764,524	(76,421)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	82,973		82,973
Issuance of treasury stock under employee stock plans (in shares)		854
Issuance of treasury stock under employee stock plans	(30,209)	$ 8	(68,659)	38,442
Repurchases of common stock (in shares)		(925)
Repurchases of common stock	(228,894)	$ (9)		(228,885)
Net income	192,124				192,124
Foreign currency translation adjustments	(13,664)					(13,664)
Ending Balance (in shares) at Sep. 30, 2019		28,944
Ending Balance at Sep. 30, 2019	$ 289,767	$ 289	$ 1,225,365	$ (2,802,450)	$ 1,956,648	$ (90,085)