MARKETABLE SECURITIES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term investments:
Amortized Cost, short-term investments	$ 50,436,000	$ 74,826,000
Gross Unrealized Gains, short-term investments	140,000	0
Gross Unrealized Losses, short-term investments	(2,000)	(270,000)
Estimated Fair Value, short-term investments	50,574,000	74,556,000
Long-term investments:
Amortized Cost, long-term investments	8,016,000	66,772,000
Gross Unrealized Gains, long-term investments	152,000	52,000
Gross Unrealized Losses, long-term investments	0	(465,000)
Estimated Fair Value, long-term investments	8,168,000	66,359,000
Total investments
Amortized Cost, total	58,452,000	141,598,000
Gross Unrealized Gains, total	292,000	52,000
Gross Unrealized Losses, total	(2,000)	(735,000)
Estimated Fair Value, total	58,742,000	140,915,000
Other-than-temporary impairment losses	0	0	$ 0
Corporate debt securities
Short-term investments:
Amortized Cost, short-term investments	46,186,000	51,184,000
Gross Unrealized Gains, short-term investments	140,000	0
Gross Unrealized Losses, short-term investments	(2,000)	(236,000)
Estimated Fair Value, short-term investments	46,324,000	50,948,000
Long-term investments:
Amortized Cost, long-term investments	8,016,000	62,530,000
Gross Unrealized Gains, long-term investments	152,000	52,000
Gross Unrealized Losses, long-term investments	0	(433,000)
Estimated Fair Value, long-term investments	8,168,000	62,149,000
U.S. treasury and government agency securities
Short-term investments:
Amortized Cost, short-term investments	2,750,000	7,647,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	0	(34,000)
Estimated Fair Value, short-term investments	2,750,000	7,613,000
Long-term investments:
Amortized Cost, long-term investments		2,742,000
Gross Unrealized Gains, long-term investments		0
Gross Unrealized Losses, long-term investments		(32,000)
Estimated Fair Value, long-term investments		2,710,000
Commercial paper
Short-term investments:
Amortized Cost, short-term investments		3,995,000
Gross Unrealized Gains, short-term investments		0
Gross Unrealized Losses, short-term investments		0
Estimated Fair Value, short-term investments		3,995,000
Certificates of deposit
Short-term investments:
Amortized Cost, short-term investments	1,500,000	12,000,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	0	0
Estimated Fair Value, short-term investments	$ 1,500,000	12,000,000
Long-term investments:
Amortized Cost, long-term investments		1,500,000
Gross Unrealized Gains, long-term investments		0
Gross Unrealized Losses, long-term investments		0
Estimated Fair Value, long-term investments		$ 1,500,000