2. STOCK-BASED COMPENSATION - Options outstanding and exercisable (Details 3) $ / shares in Units, $ in Thousands	3 Months Ended
	Aug. 31, 2015 USD ($) $ / shares shares
Range Exercise Prices Options Outstanding Minimum	$ 0.59
Range Exercise Prices Options Outstanding Maximum	$ 2.71
Number of Options Outstanding, Ending (in thousands) | shares	3,434
Weighted Average Remaining Contractual Life (Years) Options Outstanding	4 years 8 months 16 days
Weighted Average Exercise Price Outstanding, Ending	$ 1.63
Options exercisable shares, ending (in thousands) | shares	2,174
Weighted Average Remaining Contractual Life (Years) Options Exercisable	4 years 1 month 28 days
Weighted Average Exercise Price for Options exercisable, ending	$ 1.41
Aggregate Intrinsic Value for Options exercisable, ending | $	$ 2,062
Options Price Range A
Range Exercise Prices Options Outstanding Minimum	$ 0.59
Range Exercise Prices Options Outstanding Maximum	$ 0.97
Number of Options Outstanding, Ending (in thousands) | shares	670
Weighted Average Remaining Contractual Life (Years) Options Outstanding	3 years 7 months 6 days
Weighted Average Exercise Price Outstanding, Ending	$ 0.70
Options exercisable shares, ending (in thousands) | shares	627
Weighted Average Remaining Contractual Life (Years) Options Exercisable	3 years 7 months 13 days
Weighted Average Exercise Price for Options exercisable, ending	$ 0.71
Options Price Range B
Range Exercise Prices Options Outstanding Minimum	1.09
Range Exercise Prices Options Outstanding Maximum	$ 1.40
Number of Options Outstanding, Ending (in thousands) | shares	1,257
Weighted Average Remaining Contractual Life (Years) Options Outstanding	3 years 9 months
Weighted Average Exercise Price Outstanding, Ending	$ 1.28
Options exercisable shares, ending (in thousands) | shares	921
Weighted Average Remaining Contractual Life (Years) Options Exercisable	3 years 5 months 19 days
Weighted Average Exercise Price for Options exercisable, ending	$ 1.28
Options Price Range C
Range Exercise Prices Options Outstanding Minimum	1.73
Range Exercise Prices Options Outstanding Maximum	$ 2.06
Number of Options Outstanding, Ending (in thousands) | shares	267
Weighted Average Remaining Contractual Life (Years) Options Outstanding	5 years 5 months 5 days
Weighted Average Exercise Price Outstanding, Ending	$ 1.88
Options exercisable shares, ending (in thousands) | shares	216
Weighted Average Remaining Contractual Life (Years) Options Exercisable	5 years 5 months 26 days
Weighted Average Exercise Price for Options exercisable, ending	$ 1.90
Options Price Range D
Range Exercise Prices Options Outstanding Minimum	2.10
Range Exercise Prices Options Outstanding Maximum	$ 2.71
Number of Options Outstanding, Ending (in thousands) | shares	1,240
Weighted Average Remaining Contractual Life (Years) Options Outstanding	6 years 1 month 13 days
Weighted Average Exercise Price Outstanding, Ending	$ 2.45
Options exercisable shares, ending (in thousands) | shares	410
Weighted Average Remaining Contractual Life (Years) Options Exercisable	5 years 10 months 6 days
Weighted Average Exercise Price for Options exercisable, ending	$ 2.53