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	21,854		21,854
Issuance of treasury stock under employee stock plans (in shares)		466
Issuance of treasury stock under employee stock plans	(35,438)	$ 5	(56,135)	20,692
Repurchases of common stock (in shares)		(425)
Repurchases of common stock	(82,700)	$ (4)		(82,696)
Net income	40,007				40,007
Foreign currency translation adjustments	(3,265)					(3,265)
Ending Balance (in shares) at Dec. 31, 2018		29,056
Ending Balance at Dec. 31, 2018	227,895	$ 291	1,176,770	(2,674,011)	1,804,531	(79,686)
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)