Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Accumulated Deficit
Beginning balance (in shares) at Sep. 30, 2019		30,933,000
Beginning balance at Sep. 30, 2019	$ 538,754	$ 31	$ 854,275	$ (315,552)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	28,548			28,548
Net comprehensive income	28,548			28,548
Stock-based compensation expense	4,869		4,869
Exercises of stock options (in shares)		50,000
Exercises of stock options	204		204
Shares issued under employee stock plans, net (in shares)		588,000
Forfeiture and other settlements of restricted stock (in shares)		(17,000)
Forfeiture and other settlements of restricted stock	(2,058)		(2,058)
Common stock redeemed for tax liability (in shares)		(172,000)
Common stock redeemed for tax liability	(2,674)		(2,674)
Share repurchases (in shares)		(362,000)
Share repurchases	(3,327)	$ 0	(3,327)
Ending balance (in shares) at Jun. 30, 2020		31,020,000
Ending balance at Jun. 30, 2020	564,316	$ 31	851,289	(287,004)
Beginning balance (in shares) at Sep. 30, 2019		30,933,000
Beginning balance at Sep. 30, 2019	538,754	$ 31	854,275	(315,552)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share repurchases	(3,300)
Ending balance (in shares) at Sep. 30, 2020		31,012,000
Ending balance at Sep. 30, 2020	593,171	$ 31	856,466	(263,326)
Beginning balance (in shares) at Mar. 31, 2020		31,020,000
Beginning balance at Mar. 31, 2020	547,482	$ 31	849,643	(302,192)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,188			15,188
Net comprehensive income	15,188			15,188
Stock-based compensation expense	1,659		1,659
Shares issued under employee stock plans, net (in shares)		7,000
Forfeiture and other settlements of restricted stock (in shares)		(6,000)
Common stock redeemed for tax liability (in shares)		(1,000)
Common stock redeemed for tax liability	(13)		(13)
Ending balance (in shares) at Jun. 30, 2020		31,020,000
Ending balance at Jun. 30, 2020	564,316	$ 31	851,289	(287,004)
Beginning balance (in shares) at Sep. 30, 2020		31,012,000
Beginning balance at Sep. 30, 2020	593,171	$ 31	856,466	(263,326)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	73,660			73,660
Net comprehensive income	73,660			73,660
Stock-based compensation expense	9,254		9,254
Exercises of stock options (in shares)		198,000
Exercises of stock options	564		564
Shares issued under employee stock plans, net (in shares)		417,000
Forfeiture and other settlements of restricted stock (in shares)		(29,000)
Forfeiture and other settlements of restricted stock	0		0
Common stock redeemed for tax liability (in shares)		(304,000)
Common stock redeemed for tax liability	$ (3,044)		(3,044)
Share repurchases (in shares)	0
Ending balance (in shares) at Jun. 30, 2021		31,294,000
Ending balance at Jun. 30, 2021	$ 673,605	$ 31	863,240	(189,666)
Beginning balance (in shares) at Mar. 31, 2021		31,289,000
Beginning balance at Mar. 31, 2021	633,767	$ 31	860,537	(226,801)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,135			37,135
Net comprehensive income	37,135			37,135
Stock-based compensation expense	3,194		3,194
Exercises of stock options (in shares)		30,000
Exercises of stock options	(67)		(67)
Shares issued under employee stock plans, net (in shares)		6,000
Forfeiture and other settlements of restricted stock (in shares)		(13,000)
Forfeiture and other settlements of restricted stock	0		0
Common stock redeemed for tax liability (in shares)		(18,000)
Common stock redeemed for tax liability	$ (424)		(424)
Share repurchases (in shares)	0
Ending balance (in shares) at Jun. 30, 2021		31,294,000
Ending balance at Jun. 30, 2021	$ 673,605	$ 31	$ 863,240	$ (189,666)