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Share-based payments (Details)
 1 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
¥ / shares
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Jan. 31, 2018
$ / shares
shares
Jan. 31, 2017
¥ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Dec. 31, 2016
$ / shares
shares
Dec. 31, 2015
$ / shares
shares
Dec. 31, 2012
shares
Jul. 31, 2014
shares
Mar. 31, 2011
shares
Share-based Payments                      
Number of options granted to executive officers, employees and a non-employee of the Group (in shares)           1,320,000          
Maximum period of authorized leave of absence after which vesting shall be suspended         90 years            
Assumptions used in valuation of the fair value of the share options                      
Expected dividend yield (as a percent)         0.00%            
Exercise multiples         2.80            
Options outstanding                      
Outstanding at the beginning of the period (in shares)     2,738,233 1,872,941 2,738,233 1,872,941 2,436,871        
Granted (in shares)           1,320,000          
Exercised (in shares)         (356,736) (454,708) (560,930)        
Forfeited (in shares)           (3,000)          
Outstanding at the end of the period (in shares) 2,381,497 2,381,497     2,381,497 2,738,233 1,872,941 2,436,871      
Non-vested at the end of the period (in shares) 687,500 687,500     687,500            
Options vested and expected to vest at the end of the period (in shares) 2,381,497 2,381,497     2,381,497            
Exercisable at the end of the period (in shares) 1,693,997 1,693,997     1,693,997            
Weighted average exercise price per share                      
Outstanding at the beginning of the period (in dollars per share) | $ / shares     $ 33.50   $ 33.50 $ 0.78 $ 1.33        
Granted (in dollars per share) | $ / shares           68.35          
Exercised (in dollars per share) | $ / shares         1.65 1.95 1.53        
Forfeited (in dollars per share) | $ / shares           2.50          
Outstanding at the end of the period (in dollars per share) | $ / shares   $ 26.61     26.61 $ 33.50 $ 0.78 $ 1.33      
Option vested and expected to vest at the end of the period (in dollars per share) | $ / shares $ 26.61 26.61     26.61            
Exercisable at the end of the period (in dollars per share) | $ / shares 18.21 18.21     $ 18.21            
Weighted average remaining contractual years to expiry per share                      
Granted           4 years          
Exercised         2 years 9 months 7 days 3 years 9 months 4 years 7 months 10 days        
Forfeited           4 years 11 months 1 day          
Outstanding at the end of the period         3 years 18 days 4 years 1 month 21 days 4 years 11 months 19 days 5 years 10 months 21 days      
Options vested and expected to vest at the end of the period         3 years 18 days            
Exercisable at the end of the period         3 years 26 days            
Weighted average fair value at grant date                      
Options vested and expected to vest as of December 31, 2018         17.76            
Exercisable at the end of the period (in dollars per share) | $ / shares 9.58 9.58     $ 9.58            
Weighted average intrinsic value per option                      
Vested and expected to vest at the end of the period (in dollars per share) | $ / shares 27.98 27.98     27.98            
Exercisable at the end of the period (in dollars per share) | $ / shares $ 36.39 $ 36.39     $ 36.39            
Aggregate intrinsic value                      
Options vested and expected to vest at the end of the period | $ $ 66,644,636 $ 66,644,636     $ 66,644,636            
Exercisable at the end of the period | $ $ 61,644,292 $ 61,644,292     $ 61,644,292            
Binomial model                      
Assumptions used in valuation of the fair value of the share options                      
Risk-free interest rate (as a percent)         1.96%            
Expected volatility (as a percent)         61.00%            
Expected life         5 years            
Weighted average fair value of underlying ordinary shares | $ / shares         $ 25.09            
Minimum                      
Share-based Payments                      
Expiration period from grant date         5 years            
Post-termination exercise period         3 years            
Maximum                      
Share-based Payments                      
Expiration period from grant date         10 years            
Post-termination exercise period         9 years            
Key management | Binomial model                      
Share-based Payments                      
Estimated forfeiture rate (as a percent)         0.00% 0.00% 0.00%        
Employees | Binomial model                      
Share-based Payments                      
Estimated forfeiture rate (as a percent)         22.00% 18.00% 13.00%        
Share option                      
Share-based Payments                      
Number of options granted to executive officers, employees and a non-employee of the Group (in shares) 660,000 660,000                  
Options outstanding                      
Granted (in shares) 660,000 660,000                  
Weighted average exercise price per share                      
Exercised (in dollars per share) | (per share) $ 180.83 $ 26.30                  
Class A ordinary shares                      
Options outstanding                      
Exercised (in shares)         (356,736)   (560,930)        
2011 Plan | Class A ordinary shares                      
Share-based Payments                      
Number of ordinary shares authorized as stock based compensation                     7,350,000
2012 Stock Incentive Plan                      
Share-based Payments                      
Maximum aggregate number of shares that may be issued per calendar year                 1,500,000    
2012 Stock Incentive Plan | Restricted shares | Key management                      
Share-based Payments                      
Estimated forfeiture rate (as a percent)         0.00% 0.00% 0.00%        
2012 Stock Incentive Plan | Restricted shares | Employees                      
Share-based Payments                      
Estimated forfeiture rate (as a percent)         22.00% 18.00% 13.00%        
2012 Stock Incentive Plan | Class A ordinary shares                      
Share-based Payments                      
Number of ordinary shares authorized as stock based compensation                 9,000,000    
2014 Plan                      
Share-based Payments                      
Automatic increase of authorized ordinary shares                   1.50%  
2014 Plan | Senior management                      
Weighted average fair value at grant date                      
Exercised (in dollars per share) | (per share)     $ 68.35 ¥ 444.71              
2014 Plan | Share option | Senior management                      
Share-based Payments                      
Number of options granted to executive officers, employees and a non-employee of the Group (in shares)     1,320,000                
Options outstanding                      
Granted (in shares)     1,320,000                
2014 Plan | Restricted shares | Senior management                      
Share-based Payments                      
Number of options granted to executive officers, employees and a non-employee of the Group (in shares)     900,000                
Options outstanding                      
Granted (in shares)     900,000                
2014 Plan | Class A ordinary shares                      
Share-based Payments                      
Number of ordinary shares authorized as stock based compensation                   5,366,998  
2011, 2012 and 2014 Plan | Share option                      
Share-based Payments                      
Number of options granted to executive officers, employees and a non-employee of the Group (in shares)             0        
Options outstanding                      
Granted (in shares)             0