MARKETABLE SECURITIES - Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 250,652	$ 192,442
Unrealized Gains	42	8
Unrealized Losses	(286)	(651)
Aggregate Estimated Fair Value	250,408	191,799
Less than 1 year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	143,731	101,758
Unrealized Gains	18	6
Unrealized Losses	(108)	(205)
Aggregate Estimated Fair Value	143,641	101,559
1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	106,921	90,684
Unrealized Gains	24	2
Unrealized Losses	(178)	(446)
Aggregate Estimated Fair Value	106,767	90,240
Commercial paper | Less than 1 year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,302	31,899
Unrealized Gains	0	6
Unrealized Losses	(51)	(41)
Aggregate Estimated Fair Value	37,251	31,864
Corporate debt securities | Less than 1 year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	96,929	69,859
Unrealized Gains	18	0
Unrealized Losses	(57)	(164)
Aggregate Estimated Fair Value	96,890	69,695
Corporate debt securities | 1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76,909	56,162
Unrealized Gains	24	0
Unrealized Losses	(154)	(310)
Aggregate Estimated Fair Value	76,779	55,852
Securities of government sponsored entities | Less than 1 year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,500
Unrealized Gains	0
Unrealized Losses	0
Aggregate Estimated Fair Value	9,500
Securities of government sponsored entities | 1 to 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,012	34,522
Unrealized Gains	0	2
Unrealized Losses	(24)	(136)
Aggregate Estimated Fair Value	$ 29,988	$ 34,388