FAIR VALUE MEASUREMENTS - Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2016	Dec. 31, 2015
Fair value measurements
Investments in securities	$ 63,647	$ 38,624
Total financial assets	136,995	217,496
Convertible notes
Fair value measurements
Investments in securities	$ 678	635
U.S. treasury bills
Fair value measurements
Maximum investment term	12 months
Cash and cash equivalents	$ 19,997	100,996
Investments in securities	62,969	37,989
Government securities money market
Fair value measurements
Cash and cash equivalents	53,351	77,876
Level 1
Fair value measurements
Total financial assets	17,981	91,988
Level 1 \| U.S. treasury bills
Fair value measurements
Cash and cash equivalents	9,997	71,998
Investments in securities	7,984	19,990
Level 2
Fair value measurements
Total financial assets	118,336	124,873
Level 2 \| U.S. treasury bills
Fair value measurements
Cash and cash equivalents	10,000	28,998
Investments in securities	54,985	17,999
Level 2 \| Government securities money market
Fair value measurements
Cash and cash equivalents	53,351	77,876
Level 3
Fair value measurements
Total financial assets	678	635
Level 3 \| Convertible notes
Fair value measurements
Investments in securities	$ 678	$ 635

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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- Definition

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.

+ References

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Subparagraph a, f, g
-Article 7

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