FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets:
Total marketable securities	$ 93,001	$ 61,469
Level 1
Fair Value Assets:
Total marketable securities	0	0
Level 2
Fair Value Assets:
Total marketable securities	93,001	61,469
Level 3
Fair Value Assets:
Total marketable securities	0	0
Cash equivalents and Money market funds
Fair Value Assets:
Cash equivalents and Money market funds	20,155	42,771
Cash equivalents and Money market funds | Level 1
Fair Value Assets:
Cash equivalents and Money market funds	10,155	2,720
Cash equivalents and Money market funds | Level 2
Fair Value Assets:
Cash equivalents and Money market funds	10,000	40,051
Cash equivalents and Money market funds | Level 3
Fair Value Assets:
Cash equivalents and Money market funds	0	0
Municipal bonds
Fair Value Assets:
Total marketable securities	44,705	32,848
Municipal bonds | Level 1
Fair Value Assets:
Total marketable securities	0
Municipal bonds | Level 2
Fair Value Assets:
Total marketable securities		32,848
Municipal bonds | Level 3
Fair Value Assets:
Total marketable securities	0
Corporate bonds
Fair Value Assets:
Total marketable securities	48,296	28,621
Corporate bonds | Level 1
Fair Value Assets:
Total marketable securities	0	0
Corporate bonds | Level 2
Fair Value Assets:
Total marketable securities	48,296	28,621
Corporate bonds | Level 3
Fair Value Assets:
Total marketable securities	0
Investments in privately-held companies
Fair Value Assets:
Investments in privately-held companies	3,390	1,030
Investments in privately-held companies | Level 1
Fair Value Assets:
Investments in privately-held companies	0	0
Investments in privately-held companies | Level 2
Fair Value Assets:
Investments in privately-held companies	$ 0	0
Investments in privately-held companies | Level 3
Fair Value Assets:
Investments in privately-held companies		$ 1,030