Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Paid-in Capital	Accumulated Deficit
Beginning balance (in shares) at Sep. 30, 2018		33,522
Beginning balance at Sep. 30, 2018	$ 644,027	$ 34	$ 880,025	$ (236,032)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share repurchases (in shares)	(2,200)
Share repurchases	$ (24,000)
Ending balance (in shares) at Mar. 31, 2019		32,044
Ending balance at Mar. 31, 2019	529,158	$ 32	858,709	(329,583)
Beginning balance (in shares) at Sep. 30, 2018		33,522
Beginning balance at Sep. 30, 2018	644,027	$ 34	880,025	(236,032)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income and comprehensive income (loss)	(81,949)			(81,949)
Stock-based compensation expense	7,993		7,993
Exercises of stock options (in shares)		27
Exercises of stock options	278		278
Shares issued under employee stock plans, net (in shares)		914
Forfeiture and other settlements of restricted stock (in shares)		(36)
Common stock redeemed for tax liability (in shares)		(179)
Common stock redeemed for tax liability	(1,889)		(1,889)
Share repurchases (in shares)		(3,200)
Share repurchases	(34,624)	$ (3)	(34,621)
Ending balance (in shares) at Jun. 30, 2019		31,048
Ending balance at Jun. 30, 2019	533,836	$ 31	851,786	(317,981)
Beginning balance (in shares) at Sep. 30, 2018		33,522
Beginning balance at Sep. 30, 2018	644,027	$ 34	880,025	(236,032)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share repurchases	(34,600)
Ending balance (in shares) at Sep. 30, 2019		30,933
Ending balance at Sep. 30, 2019	538,754	$ 31	854,275	(315,552)
Beginning balance (in shares) at Mar. 31, 2019		32,044
Beginning balance at Mar. 31, 2019	529,158	$ 32	858,709	(329,583)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income and comprehensive income (loss)	11,602			11,602
Stock-based compensation expense	3,699		3,699
Shares issued under employee stock plans, net (in shares)		4
Forfeiture and other settlements of restricted stock (in shares)		(6)
Common stock redeemed for tax liability (in shares)		0
Common stock redeemed for tax liability	(3)		(3)
Share repurchases (in shares)		(994)
Share repurchases	(10,620)	$ (1)	(10,619)
Ending balance (in shares) at Jun. 30, 2019		31,048
Ending balance at Jun. 30, 2019	533,836	$ 31	851,786	(317,981)
Beginning balance (in shares) at Sep. 30, 2019		30,933
Beginning balance at Sep. 30, 2019	$ 538,754	$ 31	854,275	(315,552)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share repurchases (in shares)	(362)
Share repurchases	$ (3,300)
Ending balance (in shares) at Mar. 31, 2020		31,020
Ending balance at Mar. 31, 2020	547,482	$ 31	849,643	(302,192)
Beginning balance (in shares) at Sep. 30, 2019		30,933
Beginning balance at Sep. 30, 2019	538,754	$ 31	854,275	(315,552)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income and comprehensive income (loss)	28,548			28,548
Stock-based compensation expense	4,869		4,869
Exercises of stock options (in shares)		50
Exercises of stock options	204		204
Shares issued under employee stock plans, net (in shares)		588
Forfeiture and other settlements of restricted stock (in shares)		(17)
Forfeiture and other settlements of restricted stock	(2,058)		(2,058)
Common stock redeemed for tax liability (in shares)		(172)
Common stock redeemed for tax liability	(2,674)		(2,674)
Share repurchases (in shares)		(362)
Share repurchases	(3,327)		(3,327)
Ending balance (in shares) at Jun. 30, 2020		31,020
Ending balance at Jun. 30, 2020	564,316	$ 31	851,289	(287,004)
Beginning balance (in shares) at Mar. 31, 2020		31,020
Beginning balance at Mar. 31, 2020	547,482	$ 31	849,643	(302,192)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income and comprehensive income (loss)	15,188			15,188
Stock-based compensation expense	1,659		1,659
Shares issued under employee stock plans, net (in shares)		7
Forfeiture and other settlements of restricted stock (in shares)		(6)
Common stock redeemed for tax liability (in shares)		(1)
Common stock redeemed for tax liability	(13)		(13)
Ending balance (in shares) at Jun. 30, 2020		31,020
Ending balance at Jun. 30, 2020	$ 564,316	$ 31	$ 851,289	$ (287,004)