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SIGNIFICANT ACCOUNTING POLICIES - Property and equipment (Details)
12 Months Ended
Dec. 31, 2020
Buildings | Minimum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives (in years) 20 years
Buildings | Maximum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives (in years) 40 years
Motor vehicles  
Property, Plant and Equipment [Line Items]  
Estimated useful lives (in years) 5 years
Office and computer equipment | Minimum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives (in years) 3 years
Office and computer equipment | Maximum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives (in years) 10 years