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SIGNIFICANT ACCOUNTING POLICIES - Intangible assets (Details)
12 Months Ended
Dec. 31, 2020
Software | Minimum  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives (in years) 2 years
Software | Maximum  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives (in years) 10 years
Student populations | Minimum  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives (in years) 1 year 9 months 18 days
Student populations | Maximum  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives (in years) 15 years
Others | Minimum  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives (in years) 1 year 3 months 18 days
Others | Maximum  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives (in years) 10 years