MARKETABLE SECURITIES - Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 201,962	$ 215,414
Unrealized Gains		7
Unrealized Losses	(726)	(550)
Aggregate Estimated Fair Value	201,236	214,871
Less than 1
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	123,574	114,624
Unrealized Gains		7
Unrealized Losses	(302)	(128)
Aggregate Estimated Fair Value	123,272	114,503
1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	78,388	100,790
Unrealized Gains		0
Unrealized Losses	(424)	(422)
Aggregate Estimated Fair Value	77,964	100,368
Commercial paper | Less than 1
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,911	30,330
Unrealized Gains		0
Unrealized Losses	(14)	(27)
Aggregate Estimated Fair Value	6,897	30,303
Corporate debt securities | Less than 1
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86,531	64,794
Unrealized Gains		7
Unrealized Losses	(198)	(91)
Aggregate Estimated Fair Value	86,333	64,710
Corporate debt securities | 1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	78,388	70,207
Unrealized Gains		0
Unrealized Losses	(424)	(347)
Aggregate Estimated Fair Value	77,964	69,860
Securities of government sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		19,500
Unrealized Gains		0
Unrealized Losses		(10)
Aggregate Estimated Fair Value		19,490
Securities of government sponsored entities | Less than 1
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,132
Unrealized Losses	(90)
Aggregate Estimated Fair Value	$ 30,042
Securities of government sponsored entities | 1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		30,583
Unrealized Gains		0
Unrealized Losses		(75)
Aggregate Estimated Fair Value		$ 30,508