Schedule of options outstanding (Details) (Share options, Class A common shares, USD $)	12 Months Ended
	Dec. 31, 2013
2000 and 2004 stock option plans
Share-based compensation plans
Outstanding at 12/31/2013	286,800
Exercisable at 12/31/2013	286,800
2000 and 2004 stock option plans | Exercise price $5.89
Share-based compensation plans
Outstanding at 12/31/2013	86,400
Exercisable at 12/31/2013	86,400
Remaining contractual lives	4 years 10 months 24 days
Exercise prices for outstanding options $	$ 5.89
Exercise prices for exercisable options $	$ 5.89
2000 and 2004 stock option plans | Exercise price $14.70
Share-based compensation plans
Outstanding at 12/31/2013	24,500
Exercisable at 12/31/2013	24,500
Remaining contractual lives	0 years 7 months 6 days
Exercise prices for outstanding options $	$ 14.70
Exercise prices for exercisable options $	$ 14.70
2000 and 2004 stock option plans | Exercise price $28.40
Share-based compensation plans
Outstanding at 12/31/2013	15,000
Exercisable at 12/31/2013	15,000
Remaining contractual lives	1 year 8 months 12 days
Exercise prices for outstanding options $	$ 28.40
Exercise prices for exercisable options $	$ 28.40
2000 and 2004 stock option plans | Exercise price $28.50
Share-based compensation plans
Outstanding at 12/31/2013	13,000
Exercisable at 12/31/2013	13,000
Remaining contractual lives	1 year 6 months
Exercise prices for outstanding options $	$ 28.50
Exercise prices for exercisable options $	$ 28.50
2000 and 2004 stock option plans | Exercise price $34.88
Share-based compensation plans
Outstanding at 12/31/2013	3,200
Exercisable at 12/31/2013	3,200
Remaining contractual lives	2 years 2 months 12 days
Exercise prices for outstanding options $	$ 34.88
Exercise prices for exercisable options $	$ 34.88
2000 and 2004 stock option plans | Exercise price $34.90
Share-based compensation plans
Outstanding at 12/31/2013	7,600
Exercisable at 12/31/2013	7,600
Remaining contractual lives	2 years 7 months 6 days
Exercise prices for outstanding options $	$ 34.90
Exercise prices for exercisable options $	$ 34.90
2000 and 2004 stock option plans | Exercise price $35.85
Share-based compensation plans
Outstanding at 12/31/2013	15,450
Exercisable at 12/31/2013	15,450
Remaining contractual lives	4 years 8 months 12 days
Exercise prices for outstanding options $	$ 35.85
Exercise prices for exercisable options $	$ 35.85
2000 and 2004 stock option plans | Exercise price $36.50
Share-based compensation plans
Outstanding at 12/31/2013	19,350
Exercisable at 12/31/2013	19,350
Remaining contractual lives	2 years 6 months
Exercise prices for outstanding options $	$ 36.50
Exercise prices for exercisable options $	$ 36.50
2000 and 2004 stock option plans | Exercise price $42.87
Share-based compensation plans
Outstanding at 12/31/2013	7,000
Exercisable at 12/31/2013	7,000
Remaining contractual lives	2 years 10 months 24 days
Exercise prices for outstanding options $	$ 42.87
Exercise prices for exercisable options $	$ 42.87
2000 and 2004 stock option plans | Exercise price $46.08
Share-based compensation plans
Outstanding at 12/31/2013	13,150
Exercisable at 12/31/2013	13,150
Remaining contractual lives	4 years 4 months 24 days
Exercise prices for outstanding options $	$ 46.08
Exercise prices for exercisable options $	$ 46.08
2000 and 2004 stock option plans | Exercise price $51.90
Share-based compensation plans
Outstanding at 12/31/2013	11,550
Exercisable at 12/31/2013	11,550
Remaining contractual lives	4 years 2 months 12 days
Exercise prices for outstanding options $	$ 51.90
Exercise prices for exercisable options $	$ 51.90
2000 and 2004 stock option plans | Exercise price $52.51
Share-based compensation plans
Outstanding at 12/31/2013	6,750
Exercisable at 12/31/2013	6,750
Remaining contractual lives	3 years 2 months 12 days
Exercise prices for outstanding options $	$ 52.51
Exercise prices for exercisable options $	$ 52.51
2000 and 2004 stock option plans | Exercise price, additional grant, $52.51
Share-based compensation plans
Outstanding at 12/31/2013	46,350
Exercisable at 12/31/2013	46,350
Remaining contractual lives	3 years 8 months 12 days
Exercise prices for outstanding options $	$ 52.51
Exercise prices for exercisable options $	$ 52.51
2000 and 2004 stock option plans | Exercise price $52.59
Share-based compensation plans
Outstanding at 12/31/2013	8,250
Exercisable at 12/31/2013	8,250
Remaining contractual lives	3 years 6 months
Exercise prices for outstanding options $	$ 52.59
Exercise prices for exercisable options $	$ 52.59
2000 and 2004 stock option plans | Exercise price $59.23
Share-based compensation plans
Outstanding at 12/31/2013	9,250
Exercisable at 12/31/2013	9,250
Remaining contractual lives	3 years 10 months 24 days
Exercise prices for outstanding options $	$ 59.23
Exercise prices for exercisable options $	$ 59.23
2009 share award and incentive plan
Share-based compensation plans
Outstanding at 12/31/2013	2,771,500
Exercisable at 12/31/2013	526,750
2009 share award and incentive plan | Exercise price $8.38
Share-based compensation plans
Outstanding at 12/31/2013	76,500
Exercisable at 12/31/2013	76,500
Remaining contractual lives	5 years 4 months 24 days
Exercise prices for outstanding options $	$ 8.38
Exercise prices for exercisable options $	$ 8.38
2009 share award and incentive plan | Exercise price $7.71
Share-based compensation plans
Outstanding at 12/31/2013	5,000
Exercisable at 12/31/2013	5,000
Remaining contractual lives	5 years 6 months
Exercise prices for outstanding options $	$ 7.71
Exercise prices for exercisable options $	$ 7.71
2009 share award and incentive plan | Exercise price $8.91
Share-based compensation plans
Outstanding at 12/31/2013	98,200
Exercisable at 12/31/2013	98,200
Remaining contractual lives	5 years 10 months 24 days
Exercise prices for outstanding options $	$ 8.91
Exercise prices for exercisable options $	$ 8.91
2009 share award and incentive plan | Exercise price $9.93
Share-based compensation plans
Outstanding at 12/31/2013	5,000
Exercisable at 12/31/2013	5,000
Remaining contractual lives	6 years 1 month 6 days
Exercise prices for outstanding options $	$ 9.93
Exercise prices for exercisable options $	$ 9.93
2009 share award and incentive plan | Exercise price $8.37
Share-based compensation plans
Outstanding at 12/31/2013	117,400
Exercisable at 12/31/2013	117,400
Remaining contractual lives	6 years 6 months
Exercise prices for outstanding options $	$ 8.37
Exercise prices for exercisable options $	$ 8.37
2009 share award and incentive plan | Exercise price $11.44
Share-based compensation plans
Outstanding at 12/31/2013	224,650
Exercisable at 12/31/2013	224,650
Remaining contractual lives	6 years 10 months 24 days
Exercise prices for outstanding options $	$ 11.44
Exercise prices for exercisable options $	$ 11.44
2009 share award and incentive plan | Exercise price $11.69
Share-based compensation plans
Outstanding at 12/31/2013	251,500
Exercisable at 12/31/2013	0
Remaining contractual lives	7 years 4 months 24 days
Exercise prices for outstanding options $	$ 11.69
Exercise prices for exercisable options $	$ 0.00
2009 share award and incentive plan | Exercise price $8.06
Share-based compensation plans
Outstanding at 12/31/2013	520,150
Exercisable at 12/31/2013	0
Remaining contractual lives	7 years 10 months 24 days
Exercise prices for outstanding options $	$ 8.06
Exercise prices for exercisable options $	$ 0.00
2009 share award and incentive plan | Exercise price $9.95
Share-based compensation plans
Outstanding at 12/31/2013	31,700
Exercisable at 12/31/2013	0
Remaining contractual lives	8 years 2 months 12 days
Exercise prices for outstanding options $	$ 9.95
Exercise prices for exercisable options $	$ 0.00
2009 share award and incentive plan | Exercise price $8.42
Share-based compensation plans
Outstanding at 12/31/2013	283,200
Exercisable at 12/31/2013	0
Remaining contractual lives	8 years 6 months
Exercise prices for outstanding options $	$ 8.42
Exercise prices for exercisable options $	$ 0.00
2009 share award and incentive plan | Exercise price $11.32
Share-based compensation plans
Outstanding at 12/31/2013	401,550
Exercisable at 12/31/2013	0
Remaining contractual lives	8 years 10 months 24 days
Exercise prices for outstanding options $	$ 11.32
Exercise prices for exercisable options $	$ 0.00
2009 share award and incentive plan | Exercise price, additional grant, $9.95
Share-based compensation plans
Outstanding at 12/31/2013	31,700
Exercisable at 12/31/2013	0
Remaining contractual lives	9 years 1 month 9 days
Exercise prices for outstanding options $	$ 9.95
Exercise prices for exercisable options $	$ 0.00
2009 share award and incentive plan | Exercise price $11.74
Share-based compensation plans
Outstanding at 12/31/2013	289,000
Exercisable at 12/31/2013	0
Remaining contractual lives	9 years 5 months 12 days
Exercise prices for outstanding options $	$ 11.74
Exercise prices for exercisable options $	$ 0.00
2009 share award and incentive plan | Exercise price $14.51
Share-based compensation plans
Outstanding at 12/31/2013	435,950
Exercisable at 12/31/2013	0
Remaining contractual lives	9 years 11 months 12 days
Exercise prices for outstanding options $	$ 14.51
Exercise prices for exercisable options $	$ 0.00