CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Treasury Stock	Total
Beginning Balance at Sep. 30, 2017	$ 833	$ 1,874,918	$ 15,213	$ (1,082,364)	$ (200,956)	$ 607,644
Beginning Balance (in shares) at Sep. 30, 2017	83,294,848
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under stock option, restricted stock and purchase plans, net	$ 7	1,388				1,395
Shares issued under stock option, restricted stock and purchase plans, net (in shares)	706,877
Stock-based compensation		10,129				10,129
Common stock dividend declared, at $0.20 and $0 per share during the six month ended March 31, 2019 and 2018 respectively				(14,125)		(14,125)
Foreign currency translation adjustments			9,287			9,287
Actuarial gains (losses), net of tax effects of $2 and $3 during the six months ended March 31, 2019 and 2018 respectively			(3)			(3)
Net income				83,506		83,506
Ending Balance at Mar. 31, 2018	$ 840	1,886,435	24,497	(1,012,983)	(200,956)	697,833
Ending Balance (in shares) at Mar. 31, 2018	84,001,725
Beginning Balance at Sep. 30, 2018	$ 841	1,898,434	13,587	(994,074)	(200,956)	$ 717,832
Beginning Balance (in shares) at Sep. 30, 2018	84,164,130					70,702,261
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under stock option, restricted stock and purchase plans, net	$ 15	1,533				$ 1,548
Shares issued under stock option, restricted stock and purchase plans, net (in shares)	1,429,052
Stock-based compensation		9,717				9,717
Common stock dividend declared, at $0.20 and $0 per share during the six month ended March 31, 2019 and 2018 respectively				(14,428)		(14,428)
Foreign currency translation adjustments			(1,836)			(1,836)
Changes in unrealized losses on marketable securities, net of tax effects of $0			111			111
Actuarial gains (losses), net of tax effects of $2 and $3 during the six months ended March 31, 2019 and 2018 respectively			(10)			(10)
Net income				17,836		17,836
Ending Balance at Mar. 31, 2019	$ 856	$ 1,909,684	$ 11,852	(991,525)	$ (200,956)	$ 729,911
Ending Balance (in shares) at Mar. 31, 2019	85,593,182					72,131,313
Increase (Decrease) in Stockholders' Equity [Roll Forward]
ASC 606 adjustment				$ (859)		$ (859)