Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Restricted Stock Units	Performance Shares	Common Stock	Common Stock Restricted Stock Units	Common Stock Performance Shares	Additional paid in capital	Additional paid in capital Restricted Stock Units	Additional paid in capital Performance Shares	Retained earnings (accumulated deficit)	Accumulated other comprehensive loss	Treasury Stock
Beginning Balance at May. 31, 2015	$ 545,099			$ 360			$ 520,101			$ 27,575	$ (833)	$ (2,104)
Beginning Balance, Shares at May. 31, 2015				36,043,725								142,305.000
Net income	(43,590)									(43,590)
Exercise of stock options	1,297			$ 1			1,296
Exercise of stock options, Shares				101,040
Issuance/cancellation of common stock units	(331)			$ 1			(332)
Issuance/cancellation of common stock units, Shares				137,681
Purchase of common stock under Employee Stock Purchase Plan	1,471			$ 1			1,470
Purchase of common stock under Employee Stock Purchase Plan, Shares				137,957
Stock-based compensation	3,240						3,240
Other comprehensive income (loss), net of tax	42										42
Ending Balance at May. 31, 2016	507,228			$ 363			525,775			(16,015)	(791)	$ (2,104)
Ending Balance, Shares at May. 31, 2016				36,420,403.000								(142,305.000)
Net income	5,008									5,008
Exercise of stock options	9,865			$ 7			9,858
Exercise of stock options, Shares				751,062
Issuance/cancellation of common stock units		$ (586)	$ 0		$ 1	$ 0		$ (587)	$ 0
Issuance/cancellation of common stock units, Shares					158,341	23,405
Purchase of common stock under Employee Stock Purchase Plan	1,419			$ 1			1,418
Purchase of common stock under Employee Stock Purchase Plan, Shares				129,185
Stock-based compensation	6,183						6,183
Treasury stock retirement	0			$ (2)			(9,942)					$ 9,944
Treasury stock retirement, Shares				(642,305)								642,305
Common stock repurchased	(13,557)			$ 3								$ (13,554)
Common stock repurchased, Shares				(370,000)								(870,000)
Other comprehensive income (loss), net of tax	(533)										(533)
Ending Balance at May. 31, 2017	515,027			$ 367			532,705			(11,007)	(1,324)	$ (5,714)
Ending Balance, Shares at May. 31, 2017				37,210,091								370,000
Adjustment for ASU 2016-09	0						199			(199)
Net income	16,335									16,335
Exercise of stock options	$ 1,917			$ 1			1,916
Exercise of stock options, Shares	271,344			148,937
Issuance/cancellation of common stock units		$ (231)			$ 1			$ (232)
Issuance/cancellation of common stock units, Shares					145,522
Purchase of common stock under Employee Stock Purchase Plan	$ 1,263			$ 1			1,262
Purchase of common stock under Employee Stock Purchase Plan, Shares				89,943
Stock-based compensation	7,912						7,912
Other comprehensive income (loss), net of tax	372										372
Ending Balance at May. 31, 2018	$ 542,595			$ 370			$ 543,762			$ 5,129	$ (952)	$ (5,714)
Ending Balance, Shares at May. 31, 2018				37,594,493								370,000