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, 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)
Beginning Balance (in shares) at Sep. 30, 2019		28,944
Beginning 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	68,197		68,197
Issuance of treasury stock under employee stock plans (in shares)		670
Issuance of treasury stock under employee stock plans	(73,831)	$ 7	(105,845)	32,007
Repurchases of common stock (in shares)		(615)
Repurchases of common stock	(210,000)	$ (6)		(209,994)
Net income	177,285				177,285
Foreign currency translation adjustments	(4,132)					(4,132)
Ending Balance (in shares) at Jun. 30, 2020		28,999
Ending Balance at Jun. 30, 2020	247,286	$ 290	1,187,717	(2,980,437)	2,133,933	(94,217)
Beginning Balance (in shares) at Mar. 31, 2020		29,082
Beginning Balance at Mar. 31, 2020	214,151	$ 291	1,169,217	(2,930,165)	2,069,857	(95,049)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	22,264		22,264
Issuance of treasury stock under employee stock plans (in shares)		74
Issuance of treasury stock under employee stock plans	(47)	$ 1	(3,764)	3,716
Repurchases of common stock (in shares)		(157)
Repurchases of common stock	(53,990)	$ (2)		(53,988)
Net income	64,076				64,076
Foreign currency translation adjustments	832					832
Ending Balance (in shares) at Jun. 30, 2020		28,999
Ending Balance at Jun. 30, 2020	$ 247,286	$ 290	$ 1,187,717	$ (2,980,437)	$ 2,133,933	$ (94,217)