MARKETABLE SECURITIES (Schedule of the Fair Value to Amortized Cost Basis of Securities Held-to-Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term marketable securities:
Amortized Cost	$ 273,366
Gross Unrealized Gains	1
Gross Unrealized Losses	(88)
Fair Value	273,279
Long-term marketable securities:
Amortized Cost	25,032
Gross Unrealized Gains	0
Gross Unrealized Losses	(71)
Fair Value	24,961
Amortized Cost	298,398	$ 207,332
Gross Unrealized Gains	1	18
Gross Unrealized Losses	(159)	(56)
Fair Value	298,240	207,294
Commercial paper
Short-term marketable securities:
Amortized Cost	138,412	45,894
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(1)	0
Fair Value	138,412	45,894
Agency bonds
Short-term marketable securities:
Amortized Cost	78,958	152,429
Gross Unrealized Gains	0	18
Gross Unrealized Losses	(45)	(53)
Fair Value	78,913	152,394
Corporate bonds
Short-term marketable securities:
Amortized Cost	45,006	9,009
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(41)	(3)
Fair Value	44,965	$ 9,006
Long-term marketable securities:
Amortized Cost	25,032
Gross Unrealized Gains	0
Gross Unrealized Losses	(71)
Fair Value	24,961
Agency discount bonds
Short-term marketable securities:
Amortized Cost	10,990
Gross Unrealized Gains	0
Gross Unrealized Losses	(1)
Fair Value	$ 10,989