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SIGNIFICANT ACCOUNTING POLICIES - Business tax and Government subsidies (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
SIGNIFICANT ACCOUNTING POLICIES      
Government subsidy income $ 863 $ 573 $ 526
Minimum      
SIGNIFICANT ACCOUNTING POLICIES      
Business tax rate 3.00%    
Maximum      
SIGNIFICANT ACCOUNTING POLICIES      
Business tax rate 5.00%