MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 137,035	$ 189,663
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(665)	(427)
Estimated Fair Market Value	136,370	189,236
Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	38,023	38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(43)	(16)
Estimated Fair Market Value	37,980	38,469
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,387	113,742
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(242)	(216)
Estimated Fair Market Value	68,145	113,526
Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,625	37,436
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(380)	(195)
Estimated Fair Market Value	30,245	37,241
Corporate securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	38,023	38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(43)	(16)
Estimated Fair Market Value	37,980	38,469
Corporate securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	59,119	104,440
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(223)	(199)
Estimated Fair Market Value	58,896	104,241
Corporate securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,625	37,436
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(380)	(195)
Estimated Fair Market Value	30,245	37,241
Government securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,268	9,302
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(19)	(17)
Estimated Fair Market Value	$ 9,249	$ 9,285