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Summary of significant accounting policies - Guarantee liabilities and Financial guarantee derivative (Details)
12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2019
CNY (¥)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
CNY (¥)
Dec. 31, 2017
CNY (¥)
Dec. 31, 2016
CNY (¥)
Guarantee liabilities and Financial guarantee derivative            
Guarantee liabilities $ 2,510,170 ¥ 17,475,303 $ 3,001,839 ¥ 20,898,201 ¥ 545,169,033 ¥ 100,661,452
ZhongAn            
Guarantee liabilities and Financial guarantee derivative            
Period for compensation in case of default by borrowers 2 days          
Xiaoying Housing Loan            
Guarantee liabilities and Financial guarantee derivative            
Guarantee liabilities $ 18,672 ¥ 129,996 $ 229,895 ¥ 1,600,482 ¥ 8,688,973 ¥ 7,298,609