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Income taxes - Effect of Tax holiday and preferential tax rate (Details)
12 Months Ended
Dec. 31, 2019
USD ($)
$ / shares
Dec. 31, 2019
CNY (¥)
¥ / shares
Dec. 31, 2018
CNY (¥)
¥ / shares
Income taxes      
Aggregate amount of tax holiday and preferential tax rate $ 40,194,099 ¥ 279,823,276 ¥ 104,548,726
Aggregate effect on basic net income per share | (per share) $ 0.13 ¥ 0.89 ¥ 0.36
Aggregate effect on diluted net income per share | (per share) $ 0.13 ¥ 0.88 ¥ 0.34