CONDENSED CONSOLIDATED STATEMENT 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	54,631		54,631
Issuance of treasury stock under employee stock plans (in shares)		433
Issuance of treasury stock under employee stock plans	(40,380)	$ 4	(57,740)	17,356
Repurchases of common stock (in shares)		(1,359)
Repurchases of common stock	(231,479)	$ (13)		(231,466)
Net income	93,769				93,769
Foreign currency translation adjustments	(6,600)					(6,600)
Ending Balance (in shares) at Jun. 30, 2018		29,317
Ending Balance at Jun. 30, 2018	336,124	$ 293	1,192,322	(2,515,207)	1,731,811	(73,095)
Beginning Balance (in shares) at Mar. 31, 2018		29,835
Beginning Balance at Mar. 31, 2018	414,823	$ 298	1,176,905	(2,410,171)	1,702,090	(54,299)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	18,882		18,882
Issuance of treasury stock under employee stock plans (in shares)		45
Issuance of treasury stock under employee stock plans	(1,638)		(3,465)	1,827
Repurchases of common stock (in shares)		(563)
Repurchases of common stock	(106,868)	$ (5)		(106,863)
Net income	29,721				29,721
Foreign currency translation adjustments	(18,796)					(18,796)
Ending Balance (in shares) at Jun. 30, 2018		29,317
Ending Balance at Jun. 30, 2018	336,124	$ 293	1,192,322	(2,515,207)	1,731,811	(73,095)
Beginning Balance (in shares) at Sep. 30, 2018		29,015
Beginning 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	61,931		61,931
Issuance of treasury stock under employee stock plans (in shares)		696
Issuance of treasury stock under employee stock plans	(35,136)	$ 7	(66,243)	31,100
Repurchases of common stock (in shares)		(780)
Repurchases of common stock	(178,926)	$ (8)		(178,918)
Net income	137,540				137,540
Foreign currency translation adjustments	(4,529)					(4,529)
Ending Balance (in shares) at Jun. 30, 2019		28,931
Ending Balance at Jun. 30, 2019	268,317	$ 289	1,206,739	(2,759,825)	1,902,064	(80,950)
Beginning Balance (in shares) at Mar. 31, 2019		29,056
Beginning Balance at Mar. 31, 2019	247,478	$ 291	1,190,341	(2,704,305)	1,837,912	(76,761)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	19,595		19,595
Issuance of treasury stock under employee stock plans (in shares)		80
Issuance of treasury stock under employee stock plans	484	$ 1	(3,197)	3,680
Repurchases of common stock (in shares)		(205)
Repurchases of common stock	(59,203)	$ (3)		(59,200)
Net income	64,152				64,152
Foreign currency translation adjustments	(4,189)					(4,189)
Ending Balance (in shares) at Jun. 30, 2019		28,931
Ending Balance at Jun. 30, 2019	$ 268,317	$ 289	$ 1,206,739	$ (2,759,825)	$ 1,902,064	$ (80,950)