Condensed Consolidated Statements of Stockholders' Deficit - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Sep. 30, 2017	$ (363,616)	$ 1,296		$ (283,076)	$ (81,836)
Balance (in shares) at Sep. 30, 2017		129,585
Increase (Decrease) in Stockholders' Equity
Net earnings	83,264			83,264
Other comprehensive income (loss)	(1,058)				(1,058)
Repurchases and cancellations of common stock	(64,612)	$ (39)	$ (3,386)	(61,187)
Repurchases and cancellations of common stock (in shares)		(3,848)
Share-based compensation	3,111		3,111
Stock issued for stock options	276	$ 1	275
Stock issued for stock options (in shares)		62
Balance at Dec. 31, 2017	(342,635)	$ 1,258		(260,999)	(82,894)
Balance (in shares) at Dec. 31, 2017		125,799
Balance at Sep. 30, 2017	(363,616)	$ 1,296		(283,076)	(81,836)
Balance (in shares) at Sep. 30, 2017		129,585
Increase (Decrease) in Stockholders' Equity
Net earnings	144,635
Other comprehensive income (loss)	11,689
Balance at Mar. 31, 2018	(315,029)	$ 1,230		(246,112)	(70,147)
Balance (in shares) at Mar. 31, 2018		123,002
Balance at Dec. 31, 2017	(342,635)	$ 1,258		(260,999)	(82,894)
Balance (in shares) at Dec. 31, 2017		125,799
Increase (Decrease) in Stockholders' Equity
Net earnings	61,371			61,371
Other comprehensive income (loss)	12,747				12,747
Repurchases and cancellations of common stock	(50,086)	$ (29)	(3,573)	(46,484)
Repurchases and cancellations of common stock (in shares)		(2,899)
Share-based compensation	2,738		2,738
Stock issued for stock options	836	$ 1	835
Stock issued for stock options (in shares)		102
Balance at Mar. 31, 2018	(315,029)	$ 1,230		(246,112)	(70,147)
Balance (in shares) at Mar. 31, 2018		123,002
Balance at Sep. 30, 2018	$ (268,556)	$ 1,199		(179,764)	(89,991)
Balance (in shares) at Sep. 30, 2018	119,926	119,926
Increase (Decrease) in Stockholders' Equity
Net earnings	$ 65,727			65,727
Other comprehensive income (loss)	(16,705)				(16,705)
Share-based compensation	3,354		3,354
Stock issued for stock options	1,449	$ 1	1,448
Stock issued for stock options (in shares)		115
Balance at Dec. 31, 2018	(214,731)	$ 1,200	4,802	(114,037)	(106,696)
Balance (in shares) at Dec. 31, 2018		120,041
Balance at Sep. 30, 2018	$ (268,556)	$ 1,199		(179,764)	(89,991)
Balance (in shares) at Sep. 30, 2018	119,926	119,926
Increase (Decrease) in Stockholders' Equity
Net earnings	$ 131,452
Other comprehensive income (loss)	(15,563)
Balance at Mar. 31, 2019	$ (145,075)	$ 1,201	7,590	(48,312)	(105,554)
Balance (in shares) at Mar. 31, 2019	120,107	120,107
Balance at Dec. 31, 2018	$ (214,731)	$ 1,200	4,802	(114,037)	(106,696)
Balance (in shares) at Dec. 31, 2018		120,041
Increase (Decrease) in Stockholders' Equity
Net earnings	65,725			65,725
Other comprehensive income (loss)	1,142				1,142
Share-based compensation	2,517		2,517
Stock issued for stock options	272	$ 1	271
Stock issued for stock options (in shares)		66
Balance at Mar. 31, 2019	$ (145,075)	$ 1,201	$ 7,590	$ (48,312)	$ (105,554)
Balance (in shares) at Mar. 31, 2019	120,107	120,107