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	35,749		35,749
Issuance of treasury stock under employee stock plans (in shares)		388
Issuance of treasury stock under employee stock plans	(38,742)	$ 4	(54,275)	15,529
Repurchases of common stock (in shares)		(796)
Repurchases of common stock	(124,611)	$ (8)		(124,603)
Net income	64,048				64,048
Foreign currency translation adjustments	12,196					12,196
Ending Balance (in shares) at Mar. 31, 2018		29,835
Ending Balance at Mar. 31, 2018	414,823	$ 298	1,176,905	(2,410,171)	1,702,090	(54,299)
Beginning Balance (in shares) at Dec. 31, 2017		30,246
Beginning Balance at Dec. 31, 2017	430,868	$ 302	1,160,274	(2,337,205)	1,670,921	(63,424)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	19,239		19,239
Issuance of treasury stock under employee stock plans (in shares)		50
Issuance of treasury stock under employee stock plans	(569)	$ 1	(2,608)	2,038
Repurchases of common stock (in shares)		(461)
Repurchases of common stock	(75,009)	$ (5)		(75,004)
Net income	31,169				31,169
Foreign currency translation adjustments	9,125					9,125
Ending Balance (in shares) at Mar. 31, 2018		29,835
Ending Balance at Mar. 31, 2018	414,823	$ 298	1,176,905	(2,410,171)	1,702,090	(54,299)
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)