MARKETABLE SECURITIES (Schedule of the Fair Value to Amortized Cost Basis of Securities Held-to-Maturity) (Details) - Short-term marketable securities - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 195,768	$ 207,332
Gross Unrealized Gains	3	18
Gross Unrealized Losses	(58)	(56)
Fair Value	195,713	207,294
Agency bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	128,005	152,429
Gross Unrealized Gains	1	18
Gross Unrealized Losses	(53)	(53)
Fair Value	127,953	152,394
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	41,817	45,894
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	41,818	45,894
Agency discount notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,948
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	15,949
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,998	9,009
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5)	(3)
Fair Value	$ 9,993	$ 9,006