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SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Concentration of credit risk (Details)
¥ in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2018
CNY (¥)
Dec. 31, 2017
Dec. 31, 2019
CNY (¥)
Foreign currency risk        
Cash and cash equivalents $ 83,496 ¥ 853,740   ¥ 581,281
A        
Concentration of credit risk        
Concentration risk (as a percent) 18.80%      
Foreign currency risk, Renminbi ("RMB")        
Foreign currency risk        
Cash and cash equivalents   ¥ 17,810   ¥ 336,583
Accounts receivable        
Concentration of credit risk        
Concentration risk (as a percent) 10.00% 10.00% 10.00%  
Product purchases | Suppliers risk | A        
Concentration of credit risk        
Concentration risk (as a percent)   13.90% 14.10%  
Product purchases | Suppliers risk | B        
Concentration of credit risk        
Concentration risk (as a percent)   12.70%    
Accounts payable | Suppliers risk | A        
Concentration of credit risk        
Concentration risk (as a percent)   18.90%