MARKETABLE SECURITIES - Available for Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 199,176	$ 192,442
Unrealized Gains	109	8
Unrealized Losses	(202)	(651)
Aggregate Estimated Fair Value	199,083	191,799
Less than 1 year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	117,517	101,758
Unrealized Gains	14	6
Unrealized Losses	(142)	(205)
Aggregate Estimated Fair Value	117,389	101,559
1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	81,659	90,684
Unrealized Gains	95	2
Unrealized Losses	(60)	(446)
Aggregate Estimated Fair Value	81,694	90,240
Commercial paper | Less than 1 year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,939	31,899
Unrealized Gains	0	6
Unrealized Losses	(14)	(41)
Aggregate Estimated Fair Value	26,925	31,864
Corporate debt securities | Less than 1 year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	90,578	69,859
Unrealized Gains	14	0
Unrealized Losses	(128)	(164)
Aggregate Estimated Fair Value	90,464	69,695
Corporate debt securities | 1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,149	56,162
Unrealized Gains	86	0
Unrealized Losses	(52)	(310)
Aggregate Estimated Fair Value	42,183	55,852
Securities of government sponsored entities | 1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,510	34,522
Unrealized Gains	9	2
Unrealized Losses	(8)	(136)
Aggregate Estimated Fair Value	$ 39,511	$ 34,388