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Share-based compensation plans - Narratives (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
share_based_plan
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
Dec. 31, 2012
shares
Dec. 31, 2010
shares
Jun. 30, 2009
shares
Share-based compensation plans            
Number of share-based compensation plans | share_based_plan 2          
Compensation cost | $ $ 5,809 $ 6,272 $ 6,707      
Cash received from exercised options and vested awards | $ 305 $ 17 $ 35      
Total unrecognized compensation cost related to unexercised stock options and unvested share awards | $ $ 6,531          
Total unrecognized compensation cost related to unexercised stock options and unvested share awards, recognition period (in months) 30 months          
2009 share award and incentive plan | Class A common shares            
Share-based compensation plans            
Number of shares reserved for issuance           1,084,500
Increase in number of shares authorized (in shares)       5,000,000 4,000,000  
Share options | Class A common shares            
Share-based compensation plans            
Award expiration period 10 years          
Share options | 2004 stock option plans | Class A common shares            
Share-based compensation plans            
Award expiration period 10 years          
Award vesting period 3 years          
Number of shares available for grant 0          
Fair value of options exercised | $ $ 8          
Number of options vested during the period 0          
Granted (in shares) 0          
Share options | 2004 stock option plan | Class A common shares            
Share-based compensation plans            
Maximum number of options to purchase class A common shares under the plan (in shares) 1,000,000          
Number of shares reserved for issuance 15,300          
Share options | 2009 share award and incentive plan | Class A common shares            
Share-based compensation plans            
Fair value of options exercised | $ $ 505          
Number of options vested during the period 588,739          
Granted (in shares) 581,300 518,300        
Total fair value of stock options granted | $ $ 1,887          
Fair value of options that became exercisable during the period | $ $ 2,158          
Deferred shares | Class A common shares            
Share-based compensation plans            
Award expiration period 4 years          
Deferred shares | 2009 share award and incentive plan | Class A common shares            
Share-based compensation plans            
Number of shares reserved for issuance 1,271,738          
Fair value of awards issued during the period | $ $ 4,923          
Fair value of shares vested in the period | $ $ 3,925          
Options, vested and expected to vest, outstanding, number 0          
Deferred shares without performance criteria | 2009 share award and incentive plan | Class A common shares            
Share-based compensation plans            
Number of shares reserved for issuance 687,335          
Deferred shares with performance criteria | 2009 share award and incentive plan | Class A common shares            
Share-based compensation plans            
Number of shares reserved for issuance 584,403          
Restricted Stock | 2009 share award and incentive plan | Class A common shares            
Share-based compensation plans            
Options, vested and expected to vest, outstanding, number 0