MARKETABLE SECURITIES (Details) Securities in Thousands, $ in Thousands	Dec. 31, 2018 USD ($) Securities	Dec. 31, 2017 USD ($) Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 94,717	$ 189,663
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(268)	(427)
Estimated Fair Market Value	$ 94,451	$ 189,236
Weighted Average Interest Rate on Investments	2.65%	1.57%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| Securities	0
Investments due in 3 months or less:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,788	$ 38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(16)
Estimated Fair Market Value	6,786	38,469
Investments due in 4-12 months:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	60,123	113,742
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(244)	(216)
Estimated Fair Market Value	59,879	113,526
Investments due in 12 months or greater:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,806	37,436
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(22)	(195)
Estimated Fair Market Value	27,786	$ 37,241
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| Securities		0
Corporate securities \| Investments due in 3 months or less:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,788	$ 38,485
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(16)
Estimated Fair Market Value	6,786	38,469
Corporate securities \| Investments due in 4-12 months:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	60,123	104,440
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(244)	(199)
Estimated Fair Market Value	59,879	104,241
Corporate securities \| Investments due in 12 months or greater:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,806	37,436
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(22)	(195)
Estimated Fair Market Value	$ 27,786	37,241
Government securities \| Investments due in 4-12 months:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		9,302
Gross Unrealized Gains		0
Gross Unrealized Losses		(17)
Estimated Fair Market Value		$ 9,285

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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