CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Paid-in Capital	Accumulated Deficit
Beginning balance (in shares) at Sep. 30, 2017		33,516
Beginning balance at Sep. 30, 2017	$ 682,440	$ 34	$ 873,063	$ (190,657)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income and comprehensive (loss) income	(45,375)			(45,375)
Stock-based compensation expense	10,258		10,258
Exercises of stock options (in shares)		8
Exercises of stock options	64		64
Shares issued under employee stock plans, net (in shares)		443
Forfeiture and other settlements of restricted stock (in shares)		(216)
Common stock redeemed for tax liability (in shares)		(229)
Common stock redeemed for tax liability	(3,378)		(3,378)
Other activity	18		18
Ending balance (in shares) at Sep. 30, 2018		33,522
Ending balance at Sep. 30, 2018	644,027	$ 34	880,025	(236,032)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income and comprehensive (loss) income	(79,520)			(79,520)
Stock-based compensation expense	10,526		10,526
Exercises of stock options (in shares)		32
Exercises of stock options	314		314
Shares issued under employee stock plans, net (in shares)		917
Forfeiture and other settlements of restricted stock (in shares)		(68)
Common stock redeemed for tax liability (in shares)		(185)
Common stock redeemed for tax liability	(1,969)		(1,969)
Share repurchases (in shares)		(3,285)
Share repurchases	(34,624)	$ (3)	(34,621)
Ending balance (in shares) at Sep. 30, 2019		30,933
Ending balance at Sep. 30, 2019	538,754	$ 31	854,275	(315,552)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income and comprehensive (loss) income	52,226			52,226
Stock-based compensation expense	10,036		10,036
Exercises of stock options (in shares)		52
Exercises of stock options	226		226
Shares issued under employee stock plans, net (in shares)		588
Forfeiture and other settlements of restricted stock (in shares)		(26)
Forfeiture and other settlements of restricted stock	(2,058)		(2,058)
Common stock redeemed for tax liability (in shares)		(173)
Common stock redeemed for tax liability	$ (2,686)		(2,686)
Share repurchases (in shares)	(362)	(362)
Share repurchases	$ (3,327)		(3,327)
Ending balance (in shares) at Sep. 30, 2020		31,012
Ending balance at Sep. 30, 2020	$ 593,171	$ 31	$ 856,466	$ (263,326)