MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 203,889	$ 188,543
Gross Unrealized Gains	43	38
Gross Unrealized Losses	(179)	(258)
Estimated Fair Market Value	203,753	188,323
Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,802	46,338
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(14)	(2)
Estimated Fair Market Value	31,788	46,338
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	85,328	78,900
Gross Unrealized Gains	30	15
Gross Unrealized Losses	(27)	(76)
Estimated Fair Market Value	85,331	78,839
Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86,759	63,305
Gross Unrealized Gains	13	21
Gross Unrealized Losses	(138)	(180)
Estimated Fair Market Value	86,634	63,146
Corporate securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,123	9,342
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(14)	(2)
Estimated Fair Market Value	21,109	9,342
Corporate securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,313	59,714
Gross Unrealized Gains	30	15
Gross Unrealized Losses	(27)	(76)
Estimated Fair Market Value	67,316	59,653
Corporate securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,353	63,305
Gross Unrealized Gains	13	21
Gross Unrealized Losses	(127)	(180)
Estimated Fair Market Value	77,239	63,146
Commercial paper | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,679	36,996
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Market Value	10,679	36,996
Commercial paper | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,015	19,186
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Market Value	18,015	$ 19,186
Government securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,406
Gross Unrealized Gains	0
Gross Unrealized Losses	(11)
Estimated Fair Market Value	$ 9,395