Share-based compensation plans - Schedule of options outstanding (Details) - Share options - Class A common shares	12 Months Ended
	Dec. 31, 2016 $ / shares shares
2004 stock option plans
Share-based compensation plans
Outstanding (in shares)	66,350
Exercisable (in shares)	66,350
2004 stock option plans | Exercise price $5.89
Share-based compensation plans
Outstanding (in shares)	15,600
Exercisable (in shares)	15,600
Remaining contractual lives in years	1 year 10 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 5.89
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 5.89
2004 stock option plans | Exercise price $35.85
Share-based compensation plans
Outstanding (in shares)	6,550
Exercisable (in shares)	6,550
Remaining contractual lives in years	1 year 8 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 35.85
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 35.85
2004 stock option plans | Exercise price $46.08
Share-based compensation plans
Outstanding (in shares)	5,550
Exercisable (in shares)	5,550
Remaining contractual lives in years	1 year 4 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 46.08
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 46.08
2004 stock option plans | Exercise price $51.90
Share-based compensation plans
Outstanding (in shares)	4,950
Exercisable (in shares)	4,950
Remaining contractual lives in years	1 year 2 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 51.90
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 51.90
2004 stock option plans | Exercise price $52.51
Share-based compensation plans
Outstanding (in shares)	27,250
Exercisable (in shares)	27,250
Remaining contractual lives in years	8 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 52.51
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 52.51
2004 stock option plans | Exercise price $52.51
Share-based compensation plans
Outstanding (in shares)	1,050
Exercisable (in shares)	1,050
Remaining contractual lives in years	2 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 52.51
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 52.51
2004 stock option plans | Exercise price $52.59
Share-based compensation plans
Outstanding (in shares)	2,350
Exercisable (in shares)	2,350
Remaining contractual lives in years	5 months
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 52.59
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 52.59
2004 stock option plans | Exercise price $59.23
Share-based compensation plans
Outstanding (in shares)	3,050
Exercisable (in shares)	3,050
Remaining contractual lives in years	10 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 59.23
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 59.23
2009 share award and incentive plan
Share-based compensation plans
Outstanding (in shares)	2,644,489
Exercisable (in shares)	1,347,734
2009 share award and incentive plan | Exercise price $8.38
Share-based compensation plans
Outstanding (in shares)	28,600
Exercisable (in shares)	28,600
Remaining contractual lives in years	2 years 4 months 24 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 8.38
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 8.38
2009 share award and incentive plan | Exercise price $8.91
Share-based compensation plans
Outstanding (in shares)	45,700
Exercisable (in shares)	45,700
Remaining contractual lives in years	2 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
2009 share award and incentive plan | Exercise price $8.37
Share-based compensation plans
Outstanding (in shares)	42,200
Exercisable (in shares)	42,200
Remaining contractual lives in years	3 years 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
2009 share award and incentive plan | Exercise price $11.44
Share-based compensation plans
Outstanding (in shares)	85,550
Exercisable (in shares)	85,550
Remaining contractual lives in years	3 years 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
2009 share award and incentive plan | Exercise price $11.69
Share-based compensation plans
Outstanding (in shares)	65,100
Exercisable (in shares)	65,100
Remaining contractual lives in years	4 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
2009 share award and incentive plan | Exercise price $8.06
Share-based compensation plans
Outstanding (in shares)	181,750
Exercisable (in shares)	181,750
Remaining contractual lives in years	4 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
2009 share award and incentive plan | Exercise price $9.95
Share-based compensation plans
Outstanding (in shares)	31,700
Exercisable (in shares)	31,700
Remaining contractual lives in years	5 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
2009 share award and incentive plan | Exercise price $8.42
Share-based compensation plans
Outstanding (in shares)	115,500
Exercisable (in shares)	115,500
Remaining contractual lives in years	5 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
2009 share award and incentive plan | Exercise price $11.32
Share-based compensation plans
Outstanding (in shares)	198,350
Exercisable (in shares)	198,350
Remaining contractual lives in years	5 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
2009 share award and incentive plan | Exercise price $9.95
Share-based compensation plans
Outstanding (in shares)	31,700
Exercisable (in shares)	31,700
Remaining contractual lives in years	6 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
2009 share award and incentive plan | Exercise price $11.74
Share-based compensation plans
Outstanding (in shares)	103,800
Exercisable (in shares)	103,800
Remaining contractual lives in years	6 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
2009 share award and incentive plan | Exercise price $14.51
Share-based compensation plans
Outstanding (in shares)	242,950
Exercisable (in shares)	242,950
Remaining contractual lives in years	6 years 12 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 14.51
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 14.51
2009 share award and incentive plan | Exercise price $14.08
Share-based compensation plans
Outstanding (in shares)	127,100
Exercisable (in shares)	23,000
Remaining contractual lives in years	7 years 5 months 20 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 14.08
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 14.08
2009 share award and incentive plan | Exercise price $11.57
Share-based compensation plans
Outstanding (in shares)	286,183
Exercisable (in shares)	0
Remaining contractual lives in years	7 years 12 months 12 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 11.57
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 0.00
2009 share award and incentive plan | Exercise price $12.50
Share-based compensation plans
Outstanding (in shares)	96,169
Exercisable (in shares)	30,919
Remaining contractual lives in years	8 years 6 months 11 days
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 12.50
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 12.50
2009 share award and incentive plan | Exercise price $13.75
Share-based compensation plans
Outstanding (in shares)	96,165
Exercisable (in shares)	24,041
Remaining contractual lives in years	8 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
2009 share award and incentive plan | Exercise price $8.98
Share-based compensation plans
Outstanding (in shares)	375,072
Exercisable (in shares)	96,874
Remaining contractual lives in years	8 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
2009 share award and incentive plan | Exercise Price $9.64
Share-based compensation plans
Outstanding (in shares)	163,200
Exercisable (in shares)	0
Remaining contractual lives in years	9 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	$ 0.00
2009 share award and incentive plan | Exercise Price $12.75
Share-based compensation plans
Outstanding (in shares)	327,700
Exercisable (in shares)	0
Remaining contractual lives in years	9 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	$ 0.00