Share-based compensation plans - Schedule of options outstanding (Details) - Share options - Class A common shares	12 Months Ended
	Dec. 31, 2017 $ / shares shares
2004 stock option plans
Share-based compensation plans
Outstanding (in shares)	15,300
Exercisable (in shares)	15,300
2004 stock option plans | Exercise price $5.89
Share-based compensation plans
Outstanding (in shares)	8,500
Exercisable (in shares)	8,500
Remaining contractual lives in years	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)	2,600
Exercisable (in shares)	2,600
Remaining contractual lives in years	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)	2,200
Exercisable (in shares)	2,200
Remaining contractual lives in years	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)	2,000
Exercisable (in shares)	2,000
Remaining contractual lives in years	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
2009 share award and incentive plan
Share-based compensation plans
Outstanding (in shares)	2,689,407
Exercisable (in shares)	1,516,388
2009 share award and incentive plan | Exercise price $8.91
Share-based compensation plans
Outstanding (in shares)	24,800
Exercisable (in shares)	24,800
Remaining contractual lives in years	1 year 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)	13,600
Exercisable (in shares)	13,600
Remaining contractual lives in years	2 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)	54,650
Exercisable (in shares)	54,650
Remaining contractual lives in years	2 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)	44,600
Exercisable (in shares)	44,600
Remaining contractual lives in years	3 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)	132,250
Exercisable (in shares)	132,250
Remaining contractual lives in years	3 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	4 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)	50,600
Exercisable (in shares)	50,600
Remaining contractual lives in years	4 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)	152,050
Exercisable (in shares)	152,050
Remaining contractual lives in years	4 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	5 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)	82,800
Exercisable (in shares)	82,800
Remaining contractual lives in years	5 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)	184,450
Exercisable (in shares)	184,450
Remaining contractual lives in years	6 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
2009 share award and incentive plan | Exercise price $14.08
Share-based compensation plans
Outstanding (in shares)	97,100
Exercisable (in shares)	97,100
Remaining contractual lives in years	6 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
2009 share award and incentive plan | Exercise price $11.57
Share-based compensation plans
Outstanding (in shares)	251,083
Exercisable (in shares)	251,083
Remaining contractual lives in years	7 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
2009 share award and incentive plan | Exercise price $12.50
Share-based compensation plans
Outstanding (in shares)	79,000
Exercisable (in shares)	39,650
Remaining contractual lives in years	7 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
2009 share award and incentive plan | Exercise price $13.75
Share-based compensation plans
Outstanding (in shares)	96,165
Exercisable (in shares)	48,083
Remaining contractual lives in years	7 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)	332,709
Exercisable (in shares)	163,493
Remaining contractual lives in years	7 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)	152,900
Exercisable (in shares)	38,225
Remaining contractual lives in years	8 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
2009 share award and incentive plan | Exercise Price $12.75
Share-based compensation plans
Outstanding (in shares)	301,450
Exercisable (in shares)	75,554
Remaining contractual lives in years	8 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
2009 share award and incentive plan | Exercise Price $12.25
Share-based compensation plans
Outstanding (in shares)	416,700
Exercisable (in shares)	0
Remaining contractual lives in years	10 years
Exercise prices for outstanding options (in dollars per share) | $ / shares	$ 12.25
Exercise prices for exercisable options (in dollars per share) | $ / shares	$ 0
2009 share award and incentive plan | Exercise Price $13.45
Share-based compensation plans
Outstanding (in shares)	159,100
Exercisable (in shares)	0
Remaining contractual lives in years	9 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	$ 0