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SIGNIFICANT ACCOUNTING POLICIES - Significant risks and uncertainties - Foreign currency risk (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Significant risks and uncertainties    
Cash and cash equivalents $ 104,084 $ 158,691
Currency Concentration Risk | Cash and cash equivalents | RMB    
Significant risks and uncertainties    
Cash and cash equivalents $ 42,068 $ 65,375
Concentration risk, Percentage 41.00% 42.00%