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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inventories and Property and Equipment, net (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Inventories      
Inventory write-down / (reversal when sold) $ (458) $ 2,456 $ 2,065
Furniture, fixtures and office equipment | Minimum      
Property and equipment, net      
Useful lives 2 months 12 days    
Furniture, fixtures and office equipment | Maximum      
Property and equipment, net      
Useful lives 5 years    
Software and IT equipment | Minimum      
Property and equipment, net      
Useful lives 1 month 6 days    
Software and IT equipment | Maximum      
Property and equipment, net      
Useful lives 3 years    
Vehicles | Minimum      
Property and equipment, net      
Useful lives 7 months 6 days    
Vehicles | Maximum      
Property and equipment, net      
Useful lives 9 years