Share-based compensation - Schedule of management and director options activity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share compensation expenses	$ 2,252	$ 1,752
Number of options (in shares)
Outstanding (in shares)	10,981,075	10,956,838
Forfeited (in shares)	0	(324,599)
Exercised (in shares)	0	0
Outstanding (in shares)	11,329,911
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)	$ 13.23	$ 13.20
Forfeited (in dollars per share)	$ 0.00	$ 13.20
Outstanding (in dollars per share)	$ 13.38
Management
Number of options (in shares)
Granted (in shares)	348,836	348,836
Weighted average exercise price (in dollars per share)
Granted (in dollars per share)	$ 18.25	$ 14.09
Director
Number of options (in shares)
Granted (in shares)	0	0
Weighted average exercise price (in dollars per share)
Granted (in dollars per share)		$ 0.00
Management and director options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share compensation expenses	$ 1,700	$ 1,300
Number of options (in shares)
Outstanding (in shares)	11,329,911
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)	$ 13.38