STOCK-BASED COMPENSATION - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation
Total stock-based compensation expense	$ 9,833	$ 8,297	$ 6,873
Unrecognized compensation cost	11,784
Employee Stock
Stock-based compensation
Total stock-based compensation expense	73	155	129
Stock options
Stock-based compensation
Total stock-based compensation expense	593	632	789
Unrecognized compensation cost	$ 305
Weighted-Average Amortization Period (Years)	6 months
RSUs
Stock-based compensation
Total stock-based compensation expense	$ 2,282	1,920	2,492
Unrecognized compensation cost	$ 1,487
Weighted-Average Amortization Period (Years)	10 months 24 days
RSAs
Stock-based compensation
Total stock-based compensation expense	$ 4,497	3,492	2,850
Unrecognized compensation cost	$ 7,764
Weighted-Average Amortization Period (Years)	2 years 4 months 24 days
Performance-based RSAs
Stock-based compensation
Total stock-based compensation expense	$ 242	1,293	613
Market-Based RSUs
Stock-based compensation
Total stock-based compensation expense	291	112	0
Unrecognized compensation cost	$ 285
Weighted-Average Amortization Period (Years)	10 months 24 days
Market-Based RSAs
Stock-based compensation
Total stock-based compensation expense	$ 1,855	693	0
Unrecognized compensation cost	$ 1,943
Weighted-Average Amortization Period (Years)	1 year
Research and development
Stock-based compensation
Total stock-based compensation expense	$ 697	632	1,036
General and administrative
Stock-based compensation
Total stock-based compensation expense	$ 9,136	$ 7,665	$ 5,837

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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