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	42,336		42,336
Issuance of treasury stock under employee stock plans (in shares)		616
Issuance of treasury stock under employee stock plans	(35,620)	$ 6	(63,046)	27,420
Repurchases of common stock (in shares)		(575)
Repurchases of common stock	(119,723)	$ (5)		(119,718)
Net income	73,388				73,388
Foreign currency translation adjustments	(340)					(340)
Ending Balance (in shares) at Mar. 31, 2019		29,056
Ending Balance at Mar. 31, 2019	247,478	$ 291	1,190,341	(2,704,305)	1,837,912	(76,761)
Beginning Balance (in shares) at Dec. 31, 2018		29,056
Beginning Balance at Dec. 31, 2018	227,895	$ 291	1,176,770	(2,674,011)	1,804,531	(79,686)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	20,482		20,482
Issuance of treasury stock under employee stock plans (in shares)		150
Issuance of treasury stock under employee stock plans	(182)	$ 2	(6,911)	6,727
Repurchases of common stock (in shares)		(150)
Repurchases of common stock	(37,023)	$ (2)		(37,021)
Net income	33,381				33,381
Foreign currency translation adjustments	2,925					2,925
Ending Balance (in shares) at Mar. 31, 2019		29,056
Ending Balance at Mar. 31, 2019	247,478	$ 291	1,190,341	(2,704,305)	1,837,912	(76,761)
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	45,933		45,933
Issuance of treasury stock under employee stock plans (in shares)		596
Issuance of treasury stock under employee stock plans	(73,784)	$ 6	(102,081)	28,291
Repurchases of common stock (in shares)		(458)
Repurchases of common stock	(156,010)	$ (4)		(156,006)
Net income	113,209				113,209
Foreign currency translation adjustments	(4,964)					(4,964)
Ending Balance (in shares) at Mar. 31, 2020		29,082
Ending Balance at Mar. 31, 2020	214,151	$ 291	1,169,217	(2,930,165)	2,069,857	(95,049)
Beginning Balance (in shares) at Dec. 31, 2019		29,186
Beginning Balance at Dec. 31, 2019	240,961	$ 292	1,148,190	(2,843,097)	2,011,569	(75,993)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	22,788		22,788
Issuance of treasury stock under employee stock plans (in shares)		186
Issuance of treasury stock under employee stock plans	7,171	$ 2	(1,761)	8,930
Repurchases of common stock (in shares)		(290)
Repurchases of common stock	(96,001)	$ (3)		(95,998)
Net income	58,288				58,288
Foreign currency translation adjustments	(19,056)					(19,056)
Ending Balance (in shares) at Mar. 31, 2020		29,082
Ending Balance at Mar. 31, 2020	$ 214,151	$ 291	$ 1,169,217	$ (2,930,165)	$ 2,069,857	$ (95,049)