MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 221,468	$ 188,543
Gross Unrealized Gain	29	38
Gross Unrealized Losses	(151)	(258)
Estimated Fair Market Value	221,346	188,323
Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,542	46,338
Gross Unrealized Gain	0	2
Gross Unrealized Losses	(5)	(2)
Estimated Fair Market Value	42,537	46,338
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	110,313	78,900
Gross Unrealized Gain	10	15
Gross Unrealized Losses	(88)	(76)
Estimated Fair Market Value	110,235	78,839
Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,613	63,305
Gross Unrealized Gain	19	21
Gross Unrealized Losses	(58)	(180)
Estimated Fair Market Value	68,574	63,146
Corporate securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,463	9,342
Gross Unrealized Gain	0	2
Gross Unrealized Losses	(5)	(2)
Estimated Fair Market Value	24,458	9,342
Corporate securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	110,313	59,714
Gross Unrealized Gain	10	15
Gross Unrealized Losses	(88)	(76)
Estimated Fair Market Value	110,235	59,653
Corporate securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	59,241	63,305
Gross Unrealized Gain	19	21
Gross Unrealized Losses	(52)	(180)
Estimated Fair Market Value	59,208	63,146
Commercial paper | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,079	36,996
Gross Unrealized Gain	0	0
Gross Unrealized Losses	0	0
Estimated Fair Market Value	18,079	36,996
Commercial paper | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		19,186
Gross Unrealized Gain		0
Gross Unrealized Losses		0
Estimated Fair Market Value		$ 19,186
Government securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,372
Gross Unrealized Gain	0
Gross Unrealized Losses	(6)
Estimated Fair Market Value	$ 9,366