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, 2017		30,243
Beginning 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	93,681		93,681
Issuance of treasury stock under employee stock plans (in shares)		827
Issuance of treasury stock under employee stock plans	(60,644)	$ 9	(100,463)	39,810
Repurchases of common stock (in shares)		(675)
Repurchases of common stock	(235,223)	$ (7)		(235,216)
Net income	236,411				236,411
Foreign currency translation adjustments	7,090					7,090
Ending Balance (in shares) at Sep. 30, 2020		29,096
Ending Balance at Sep. 30, 2020	$ 331,082	$ 291	$ 1,218,583	$ (2,997,856)	$ 2,193,059	$ (82,995)