MARKETABLE SECURITIES (Details) Securities in Thousands, $ in Thousands	Dec. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 189,663	$ 188,543
Gross Unrealized Gains	0	38
Gross Unrealized Losses	(427)	(258)
Estimated Fair Market Value	$ 189,236	$ 188,323
Weighted Average Interest Rate on Investments	1.57%	1.23%
Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 38,485	$ 46,338
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(16)	(2)
Estimated Fair Market Value	38,469	46,338
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,742	78,900
Gross Unrealized Gains	0	15
Gross Unrealized Losses	(216)	(76)
Estimated Fair Market Value	113,526	78,839
Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,436	63,305
Gross Unrealized Gains	0	21
Gross Unrealized Losses	(195)	(180)
Estimated Fair Market Value	$ 37,241	$ 63,146
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | Securities	0	0
Commercial paper | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 36,996
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Market Value		36,996
Commercial paper | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		19,186
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Market Value		19,186
Corporate securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 38,485	9,342
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(16)	(2)
Estimated Fair Market Value	38,469	9,342
Corporate securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	104,440	59,714
Gross Unrealized Gains	0	15
Gross Unrealized Losses	(199)	(76)
Estimated Fair Market Value	104,241	59,653
Corporate securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,436	63,305
Gross Unrealized Gains	0	21
Gross Unrealized Losses	(195)	(180)
Estimated Fair Market Value	37,241	$ 63,146
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