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Share-based compensation plans - Schedule of options outstanding (Details) - 2009 share award and incentive plan - Share options - Class A common shares
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Share-based compensation plans  
Outstanding (in shares) 2,260,062
Exercisable (in shares) 1,595,372
Exercise price $8.91  
Share-based compensation plans  
Outstanding (in shares) 19,800
Exercisable (in shares) 19,800
Remaining contractual lives in years 10 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 8.91
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 8.91
Exercise price $8.37  
Share-based compensation plans  
Outstanding (in shares) 13,600
Exercisable (in shares) 13,600
Remaining contractual lives in years 1 year 5 months
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 8.37
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 8.37
Exercise price $11.44  
Share-based compensation plans  
Outstanding (in shares) 44,650
Exercisable (in shares) 44,650
Remaining contractual lives in years 1 year 10 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 11.44
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 11.44
Exercise price $11.69  
Share-based compensation plans  
Outstanding (in shares) 44,600
Exercisable (in shares) 44,600
Remaining contractual lives in years 2 years 4 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 11.69
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 11.69
Exercise price $8.06  
Share-based compensation plans  
Outstanding (in shares) 119,250
Exercisable (in shares) 119,250
Remaining contractual lives in years 2 years 10 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 8.06
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 8.06
Exercise price $9.95  
Share-based compensation plans  
Outstanding (in shares) 31,700
Exercisable (in shares) 31,700
Remaining contractual lives in years 3 years 2 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 9.95
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 9.95
Exercise price $8.42  
Share-based compensation plans  
Outstanding (in shares) 50,600
Exercisable (in shares) 50,600
Remaining contractual lives in years 3 years 5 months
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 8.42
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 8.42
Exercise price $11.32  
Share-based compensation plans  
Outstanding (in shares) 132,050
Exercisable (in shares) 132,050
Remaining contractual lives in years 3 years 10 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 11.32
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 11.32
Exercise price $9.95  
Share-based compensation plans  
Outstanding (in shares) 31,700
Exercisable (in shares) 31,700
Remaining contractual lives in years 4 years 1 month 9 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 9.95
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 9.95
Exercise price $11.74  
Share-based compensation plans  
Outstanding (in shares) 70,800
Exercisable (in shares) 70,800
Remaining contractual lives in years 4 years 5 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 11.74
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 11.74
Exercise price $14.51  
Share-based compensation plans  
Outstanding (in shares) 145,850
Exercisable (in shares) 145,850
Remaining contractual lives in years 5 years
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 14.51
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 14.51
Exercise price $14.08  
Share-based compensation plans  
Outstanding (in shares) 78,000
Exercisable (in shares) 78,000
Remaining contractual lives in years 5 years 6 months
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 14.08
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 14.08
Exercise price $11.57  
Share-based compensation plans  
Outstanding (in shares) 206,033
Exercisable (in shares) 206,033
Remaining contractual lives in years 6 years
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 11.57
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 11.57
Exercise price $12.50  
Share-based compensation plans  
Outstanding (in shares) 68,100
Exercisable (in shares) 50,575
Remaining contractual lives in years 6 years 6 months
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 12.50
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 12.50
Exercise price $13.75  
Share-based compensation plans  
Outstanding (in shares) 96,165
Exercisable (in shares) 72,124
Remaining contractual lives in years 6 years 8 months 20 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 13.75
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 13.75
Exercise price $8.98  
Share-based compensation plans  
Outstanding (in shares) 249,452
Exercisable (in shares) 182,785
Remaining contractual lives in years 6 years 10 months 20 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 8.98
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 8.98
Exercise Price $9.64  
Share-based compensation plans  
Outstanding (in shares) 123,000
Exercisable (in shares) 61,500
Remaining contractual lives in years 7 years 5 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 9.64
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 9.64
Exercise Price $12.75  
Share-based compensation plans  
Outstanding (in shares) 241,475
Exercisable (in shares) 116,675
Remaining contractual lives in years 7 years 10 months 19 days
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 12.75
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 12.75
Exercise Price $13.45  
Share-based compensation plans  
Outstanding (in shares) 138,600
Exercisable (in shares) 34,650
Remaining contractual lives in years 8 years 6 months
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 13.45
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 13.45
Exercise Price $12.25  
Share-based compensation plans  
Outstanding (in shares) 354,637
Exercisable (in shares) 88,430
Remaining contractual lives in years 9 years
Exercise prices for outstanding options (in dollars per share) | $ / shares $ 12.25
Exercise prices for exercisable options (in dollars per share) | $ / shares $ 12.25