3. CASH, CASH EQUIVALENTS AND INVESTMENTS (Details) - USD ($) $ in Thousands	May 31, 2018	May 31, 2017	May 31, 2016	May 31, 2015
Cash, cost	$ 3,132	$ 2,287
Cash, estimated fair value	3,132	2,287
Cash equivalents, cost	13,716	15,516
Cash equivalents, estimated fair value	13,716	15,516
Cash and cash equivalents, cost	16,848	17,803	$ 939	$ 5,527
Cash and cash equivalents, estimated fair value	16,848	17,803	$ 939	$ 5,527
Long-term investments, certificate of deposit, cost	80	50
Long-term investments, certificate of deposit, estimed fair value	80	50
Cash, cash equivalents, and investments, cost	16,928	17,853
Cash, cash equivalents, and investments, estimated fair value	16,928	17,853
Money Market Funds
Cash equivalents, cost	7,733	15,516
Cash equivalents, estimated fair value	7,733	$ 15,516
U.S. Treasury securities
Cash equivalents, cost	5,983
Cash equivalents, estimated fair value	$ 5,983