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Summary of significant accounting policies - Long term investments (Details)
12 Months Ended
Dec. 31, 2019
entity
Dec. 31, 2018
CNY (¥)
Dec. 31, 2017
CNY (¥)
Long-term investments      
Number of significant influence entities held | entity 2    
Acquired equity interest through nominee arrangement      
Long-term investments      
Payments to acquire equity interest     ¥ 40,000,000
Equity interest acquired (in percentage)     40.00%
Jiangxi Ruijing      
Long-term investments      
Payments to acquire equity interest   ¥ 225,000,000  
Equity interest acquired (in percentage)   15.00%  
Private entities      
Long-term investments      
Payments to acquire equity interest     ¥ 15,000,000
Equity interest acquired (in percentage)     10.00%