FAIR VALUE MEASUREMENTS - Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Fair value measurements
Investments in securities	$ 33,383	$ 38,624
Total financial assets	$ 280,316	217,496
Convertible notes
Fair value measurements
Investments in securities		635
U.S. treasury bills
Fair value measurements
Maximum investment term	12 months
Cash and cash equivalents	$ 44,999	100,996
Investments in securities	32,995	37,989
Government securities money market
Fair value measurements
Cash and cash equivalents	201,934	77,876
Common Stock
Fair value measurements
Investments in securities	388
Level 1
Fair value measurements
Total financial assets	388	91,988
Level 1 \| U.S. treasury bills
Fair value measurements
Cash and cash equivalents		71,998
Investments in securities		19,990
Level 1 \| Common Stock
Fair value measurements
Investments in securities	388
Level 2
Fair value measurements
Total financial assets	279,928	124,873
Level 2 \| U.S. treasury bills
Fair value measurements
Cash and cash equivalents	44,999	28,998
Investments in securities	32,995	17,999
Level 2 \| Government securities money market
Fair value measurements
Cash and cash equivalents	$ 201,934	77,876
Level 3
Fair value measurements
Total financial assets		635
Level 3 \| Convertible notes
Fair value measurements
Investments in securities		$ 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=66048111&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
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-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

No definition available.

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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
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6505594

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