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CASH, CASH EQUIVALENTS AND SHORT AND LONG TERM INVESTMENTS - Summary of Available-for-sale Investments (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |
Cost | $ 2,179 | $ 6,479
Cash Collection | (479) |
Impairment charges and equity losses | (1,308) | (4,300)
Transfer-out from available-for-sale investments | (392) |
Estimated fair value |  | 2,179
Level 3 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated fair value | $ 0 |
Minimum |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Maturity period of bank notes receivable | 3 months |
Maximum |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Maturity period of bank notes receivable | 1 year |
Shares of a private company |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost | $ 479 | 479
Cash Collection | (479) |
Estimated fair value |  | 479
Convertible bonds of private company |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost | 800 | 800
Impairment charges and equity losses | (408) |
Transfer-out from available-for-sale investments | (392) |
Estimated fair value |  | 800
Preferred convertible shares of private company |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost | 900 | 5,200
Impairment charges and equity losses | $ (900) | (4,300)
Estimated fair value |  | $ 900

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