MARKETABLE SECURITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Amortized Cost
Amortized cost	$ 218,070	$ 173,014
Gross Unrealized Gains
Gross Unrealized Gains	31	88
Gross Unrealized Losses
Gross Unrealized Losses	(320)	(79)
Fair Value
Fair Value	217,781	173,023
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	1,500	1,400
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	48,871	44,580
Maturing after one year through three years	15,940	16,108
Amortized cost	64,811	60,688
Gross Unrealized Gains
Maturing in one year or less	4	30
Maturing after one year through three years	24	49
Gross Unrealized Gains	28	79
Gross Unrealized Losses
Maturing in one year or less	(20)	(6)
Maturing after one year through three years	(57)	(1)
Gross Unrealized Losses	(77)	(7)
Fair Value
Maturing in one year or less	48,855	44,604
Maturing after one year through three years	15,907	16,156
Fair Value	64,762	60,760
Corporate debt securities
Amortized Cost
Maturing in one year or less	129,327	89,636
Maturing after one year through three years	23,932	22,690
Amortized cost	153,259	112,326
Gross Unrealized Gains
Maturing in one year or less	2	6
Maturing after one year through three years	1	3
Gross Unrealized Gains	3	9
Gross Unrealized Losses
Maturing in one year or less	(141)	(37)
Maturing after one year through three years	(102)	(35)
Gross Unrealized Losses	(243)	(72)
Fair Value
Maturing in one year or less	129,188	89,605
Maturing after one year through three years	23,831	22,658
Fair Value	$ 153,019	$ 112,263