CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - 6 months ended Mar. 31, 2018 - 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	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	59,574				59,574
Foreign currency translation adjustments	12,196					12,196
Ending Balance (in shares) at Mar. 31, 2018		29,835
Ending Balance at Mar. 31, 2018	$ 370,703	$ 298	$ 1,176,905	$ (2,410,171)	$ 1,657,969	$ (54,298)