MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 98,933	$ 189,663
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(439)	(427)
Estimated Fair Market Value	98,494	189,236
Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	57,160	38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(63)	(16)
Estimated Fair Market Value	57,097	38,469
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,981	113,742
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(133)	(216)
Estimated Fair Market Value	21,848	113,526
Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,792	37,436
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(243)	(195)
Estimated Fair Market Value	19,549	37,241
Corporate securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,926	38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(58)	(16)
Estimated Fair Market Value	47,868	38,469
Corporate securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,981	104,440
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(133)	(199)
Estimated Fair Market Value	21,848	104,241
Corporate securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,792	37,436
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(243)	(195)
Estimated Fair Market Value	19,549	37,241
Government securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,234
Gross Unrealized Gains	0
Gross Unrealized Losses	(5)
Estimated Fair Market Value	$ 9,229
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