CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - 3 months ended Dec. 31, 2017 - 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	$ 426,537	$ 302	$ 1,195,431	$ (2,301,097)	$ 1,598,395	$ (66,494)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	16,510		16,510
Issuance of treasury stock under employee stock plans (in shares)		338
Issuance of treasury stock under employee stock plans	(38,174)	$ 3	(51,667)	13,490
Repurchases of common stock (in shares)		(335)
Repurchases of common stock	(49,601)	$ (3)		(49,598)
Net income	27,299				27,299
Foreign currency translation adjustments	3,071					3,071
Ending Balance (in shares) at Dec. 31, 2017		30,246
Ending Balance at Dec. 31, 2017	$ 385,642	$ 302	$ 1,160,274	$ (2,337,205)	$ 1,625,694	$ (63,423)