AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS - Available-for-Sale Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Available-for-Sale Securities
Amortized Cost	$ 108,977,000	$ 122,727,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3,000)	(18,000)
Estimated Fair Value	$ 108,974,000	122,709,000
Maturity period for marketable securities
Maximum contractual maturity period	1 year
Average maturity	3 months
U.S. government securities
Available-for-Sale Securities
Amortized Cost	$ 29,736,000	9,943,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3,000)	(1,000)
Estimated Fair Value	29,733,000	9,942,000
U.S. government agencies
Available-for-Sale Securities
Amortized Cost	4,971,000	9,987,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(2,000)
Estimated Fair Value	4,971,000	9,985,000
U.S. corporate notes
Available-for-Sale Securities
Amortized Cost	2,875,000	10,881,000.000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(15,000)
Estimated Fair Value	2,875,000	10,866,000.000
U.S. commercial paper
Available-for-Sale Securities
Amortized Cost	22,037,000	29,945,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	22,037,000	29,945,000
Money market funds
Available-for-Sale Securities
Amortized Cost	49,358,000	61,971,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 49,358,000	$ 61,971,000