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Share-based compensation plans - Fair value assumptions for share-based compensation plans (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based compensation plans      
Expected dividends (usd per share) $ 0.00 $ 0.00 $ 0.00
Share options      
Share-based compensation plans      
Expected share price volatility, minimum (percentage) 27.00% 29.00% 28.00%
Expected share price volatility, maximum (percentage) 34.00% 40.00% 58.00%
Risk-free interest rate, minimum (percentage) 1.50% 0.76% 0.65%
Risk-free interest rate, maximum (percentage) 2.14% 1.84% 1.83%
Share options | Minimum      
Share-based compensation plans      
Expected life of awards (in years) 2 years 6 months 2 years 6 months 2 years 6 months
Share options | Maximum      
Share-based compensation plans      
Expected life of awards (in years) 5 years 6 months 5 years 6 months 6 years 10 months 24 days
Share options | Class A common shares | Vesting December 9, 2018 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 104,425    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 12.25    
Expected share price volatility (as a percent) 27.00%    
Risk-free interest rate (as a percent) 1.92%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 2 years 6 months    
Share options | Class A common shares | Vesting December 9, 2019 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 104,425    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 12.25    
Expected share price volatility (as a percent) 27.00%    
Risk-free interest rate (as a percent) 1.92%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 3 years 6 months    
Share options | Class A common shares | Vesting December 9, 2020 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 104,425    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 12.25    
Expected share price volatility (as a percent) 28.00%    
Risk-free interest rate (as a percent) 2.14%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 4 years 6 months    
Share options | Class A common shares | Vesting December 9, 2021 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 104,425    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 12.25    
Expected share price volatility (as a percent) 31.00%    
Risk-free interest rate (as a percent) 2.14%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 5 years 6 months    
Share options | Class A common shares | Vesting June 11, 2018 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 40,900    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 13.45    
Expected share price volatility (as a percent) 29.00%    
Risk-free interest rate (as a percent) 1.50%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 2 years 6 months    
Share options | Class A common shares | Vesting June 11, 2019 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 40,900    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 13.45    
Expected share price volatility (as a percent) 29.00%    
Risk-free interest rate (as a percent) 1.50%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 3 years 6 months    
Share options | Class A common shares | Vesting June 11, 2020 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 40,900    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 13.45    
Expected share price volatility (as a percent) 30.00%    
Risk-free interest rate (as a percent) 1.77%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 4 years 6 months    
Share options | Class A common shares | Vesting June 11, 2021 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 40,900    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 13.45    
Expected share price volatility (as a percent) 34.00%    
Risk-free interest rate (as a percent) 1.77%    
Expected dividends (usd per share) $ 0.00    
Expected life of awards (in years) 5 years 6 months    
Restricted Stock without Performance Criteria Share-based Payments | Vesting June 11, 2018 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 56,390    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01    
Restricted Stock without Performance Criteria Share-based Payments | Vesting June 11, 2020 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 48,872    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01    
Restricted Stock without Performance Criteria Share-based Payments | On Retirement | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 11,278    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01    
Restricted Stock without Performance Criteria Share-based Payments | Vesting March 17, 2018 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 34,450    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01    
Restricted Stock without Performance Criteria Share-based Payments | Vesting March 17, 2019 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 34,450    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01    
Restricted Stock without Performance Criteria Share-based Payments | Vesting March 17, 2020 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 117,231    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01    
Restricted Stock without Performance Criteria Share-based Payments | Vesting March 17, 2021 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 34,450    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01    
Restricted Stock with Performance Criteria Share-based Payments | Vesting March 17, 2020 | 2009 share award and incentive plan      
Share-based compensation plans      
Granted (in shares) 228,500    
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share) $ 0.01