SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
Buildings
Property, Plant and Equipment [Line Items]
Estimated useful lives, property and equipment	20 years
Furniture, fixtures and equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives, property and equipment	3 years
Furniture, fixtures and equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives, property and equipment	10 years
Motor vehicles | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives, property and equipment	3 years
Motor vehicles | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives, property and equipment	5 years