FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets:
Total marketable securities		$ 93,001
Level 1
Fair Value Assets:
Total marketable securities		0
Level 2
Fair Value Assets:
Total marketable securities		93,001
Level 3
Fair Value Assets:
Total marketable securities		0
Cash equivalents
Fair Value Assets:
Cash equivalents		20,155
Cash equivalents | Level 1
Fair Value Assets:
Cash equivalents		10,155
Cash equivalents | Level 2
Fair Value Assets:
Cash equivalents		10,000
Cash equivalents | Level 3
Fair Value Assets:
Cash equivalents		0
Municipal bonds
Fair Value Assets:
Total marketable securities		44,705
Municipal bonds | Level 1
Fair Value Assets:
Total marketable securities		0
Municipal bonds | Level 2
Fair Value Assets:
Total marketable securities		44,705
Municipal bonds | Level 3
Fair Value Assets:
Total marketable securities		0
Corporate bonds
Fair Value Assets:
Total marketable securities		48,296
Corporate bonds | Level 1
Fair Value Assets:
Total marketable securities		0
Corporate bonds | Level 2
Fair Value Assets:
Total marketable securities		48,296
Corporate bonds | Level 3
Fair Value Assets:
Total marketable securities		0
Investments in privately-held companies
Fair Value Assets:
Investments in privately-held companies	$ 4,871	3,390
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	$ 4,871	$ 3,390