2. STOCK-BASED COMPENSATION - Options outstanding and exercisable (Details 4) $ / shares in Units, $ in Thousands	9 Months Ended
	Feb. 29, 2016 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	3,286
Weighted Average Remaining Contractual Life (Years) Options Outstanding	4 years 3 months 25 days
Weighted Average Exercise Price Outstanding, Ending	$ 1.66
Options exercisable shares, ending (in thousands) | shares	2,339
Weighted Average Remaining Contractual Life (Years) Options Exercisable	3 years 10 months 13 days
Weighted Average Exercise Price for Options exercisable, ending	$ 1.47
Aggregate Intrinsic Value for Options exercisable, ending | $	$ 293
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	578
Weighted Average Remaining Contractual Life (Years) Options Outstanding	3 years 25 days
Weighted Average Exercise Price Outstanding, Ending	$ 0.68
Options exercisable shares, ending (in thousands) | shares	577
Weighted Average Remaining Contractual Life (Years) Options Exercisable	3 years 25 days
Weighted Average Exercise Price for Options exercisable, ending	$ 0.68
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	1,214
Weighted Average Remaining Contractual Life (Years) Options Outstanding	3 years 4 months 13 days
Weighted Average Exercise Price Outstanding, Ending	$ 1.28
Options exercisable shares, ending (in thousands) | shares	1,001
Weighted Average Remaining Contractual Life (Years) Options Exercisable	3 years 2 months 5 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	248
Weighted Average Remaining Contractual Life (Years) Options Outstanding	4 years 11 months 12 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 7 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	1,246
Weighted Average Remaining Contractual Life (Years) Options Outstanding	5 years 8 months 12 days
Weighted Average Exercise Price Outstanding, Ending	$ 2.43
Options exercisable shares, ending (in thousands) | shares	545
Weighted Average Remaining Contractual Life (Years) Options Exercisable	5 years 6 months 18 days
Weighted Average Exercise Price for Options exercisable, ending	$ 2.47