MARKETABLE SECURITIES (Schedule of the Fair Value to Amortized Cost Basis of Securities Held-to-Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Short-term marketable securities:
Amortized Cost	$ 422,283	$ 273,366
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(395)	(88)
Fair Value	421,889	273,279
Long-term marketable securities:
Amortized Cost	14,898	25,032
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(26)	(71)
Fair Value	14,872	24,961
Amortized Cost	437,181	298,398
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(421)	(159)
Fair Value	436,761	298,240
Commercial paper
Short-term marketable securities:
Amortized Cost	227,630	138,412
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(12)	(1)
Fair Value	227,619	138,412
Corporate bonds
Short-term marketable securities:
Amortized Cost	86,977	45,006
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(282)	(41)
Fair Value	86,695	44,965
Long-term marketable securities:
Amortized Cost	2,986	25,032
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9)	(71)
Fair Value	2,977	24,961
Agency bonds
Short-term marketable securities:
Amortized Cost	69,036	78,958
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(70)	(45)
Fair Value	68,966	78,913
Long-term marketable securities:
Amortized Cost	11,912
Gross Unrealized Gains	0
Gross Unrealized Losses	(17)
Fair Value	11,895
U.S. government bonds
Short-term marketable securities:
Amortized Cost	38,640
Gross Unrealized Gains	0
Gross Unrealized Losses	(31)
Fair Value	$ 38,609
Agency discount bonds
Short-term marketable securities:
Amortized Cost		10,990
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Fair Value		$ 10,989