INVESTMENTS (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-Sale, Amortized cost	$ 190,137	$ 43,176
Available-for-Sale, Gross unrealized gains	26	0
Available-for-Sale, Gross unrealized losses	(67)	(42)
Available-for-Sale, Estimated fair value	$ 190,096	$ 43,134
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, maximum maturity period	1 year	1 year
Available-for-Sale, Amortized cost	$ 111,933	$ 24,264
Available-for-Sale, Gross unrealized gains	26	0
Available-for-Sale, Gross unrealized losses	(43)	(23)
Available-for-Sale, Estimated fair value	$ 111,916	$ 24,241
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, maximum maturity period	1 year	1 year
Available-for-Sale, Amortized cost	$ 74,433	$ 18,912
Available-for-Sale, Gross unrealized gains	0	0
Available-for-Sale, Gross unrealized losses	(24)	(19)
Available-for-Sale, Estimated fair value	74,409	$ 18,893
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Available-for-Sale, Amortized cost	3,771
Available-for-Sale, Gross unrealized gains	0
Available-for-Sale, Gross unrealized losses	0
Available-for-Sale, Estimated fair value	$ 3,771

Available-for-sale Securities, Maximum Maturity Period

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- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (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 (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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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