AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS - Available-for-Sale Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Available-for-Sale Securities
Amortized Cost	$ 122,727	$ 148,812
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(18)	0
Estimated Fair Value	$ 122,709	148,813
Maturity period for marketable securities
Maximum contractual maturity period	1 year
Average maturity	3 months
U.S. government securities
Available-for-Sale Securities
Amortized Cost	$ 9,943
Gross Unrealized Gains	0
Gross Unrealized Losses	(1)
Estimated Fair Value	9,942
U.S. government agencies
Available-for-Sale Securities
Amortized Cost	9,987	12,428
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(2)	0
Estimated Fair Value	9,985	12,429
U.S. corporate notes
Available-for-Sale Securities
Amortized Cost	10,881
Gross Unrealized Gains	0
Gross Unrealized Losses	(15)
Estimated Fair Value	10,866
U.S. commercial papers
Available-for-Sale Securities
Amortized Cost	29,945	72,065
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	29,945	72,065
Money market funds
Available-for-Sale Securities
Amortized Cost	61,971	64,319
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 61,971	$ 64,319

Represents the average duration period for marketable debt securities.

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- Definition

Represents the maximum maturity period for marketable debt securities.

+ References

No definition available.

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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- Definition

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://www.xbrl.org/2003/role/presentationRef
-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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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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No definition available.

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