MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 100,040	$ 189,663
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(274)	(427)
Estimated Fair Market Value	99,766	189,236
Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,038	38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(16)
Estimated Fair Market Value	10,036	38,469
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,942	113,742
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(250)	(216)
Estimated Fair Market Value	65,692	113,526
Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,060	37,436
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(22)	(195)
Estimated Fair Market Value	24,038	37,241
Commercial paper | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,687
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Market Value	5,687
Corporate securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,351	38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(16)
Estimated Fair Market Value	4,349	38,469
Corporate securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,942	104,440
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(250)	(199)
Estimated Fair Market Value	65,692	104,241
Corporate securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,060	37,436
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(22)	(195)
Estimated Fair Market Value	$ 24,038	37,241
Government securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		9,302
Gross Unrealized Gains		0
Gross Unrealized Losses		(17)
Estimated Fair Market Value		$ 9,285