CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Loss
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	23,145		23,145
Issuance of treasury stock under employee stock plans (in shares)		410
Issuance of treasury stock under employee stock plans	(80,955)	$ 4	(100,320)	19,361
Repurchases of common stock (in shares)		(168)
Repurchases of common stock	(60,009)	$ (1)		(60,008)
Net income	54,921				54,921
Foreign currency translation adjustments	14,092					14,092
Ending Balance (in shares) at Dec. 31, 2019		29,186
Ending Balance at Dec. 31, 2019	240,961	$ 292	1,148,190	(2,843,097)	2,011,569	(75,993)
Beginning Balance (in shares) at Sep. 30, 2020		29,096
Beginning Balance at Sep. 30, 2020	331,082	$ 291	1,218,583	(2,997,856)	2,193,059	(82,995)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	25,132		25,132
Issuance of treasury stock under employee stock plans (in shares)		241
Issuance of treasury stock under employee stock plans	(85,622)	$ 2	(97,822)	12,198
Repurchases of common stock (in shares)		(101)
Repurchases of common stock	(50,011)	$ (1)		(50,010)
Net income	86,492				86,492
Foreign currency translation adjustments	17,048					17,048
Ending Balance (in shares) at Dec. 31, 2020		29,236
Ending Balance at Dec. 31, 2020	$ 324,121	$ 292	$ 1,145,893	$ (3,035,668)	$ 2,279,551	$ (65,947)