Share-based compensation - Schedule of management and director options activity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share compensation expenses	$ 4,143	$ 1,714	$ 7,611	$ 4,595
Number of options (in shares)
Outstanding (in shares)			10,956,838	0	0
Forfeited (in shares)			(290,699)		0
Exercised (in shares)			0		0
Outstanding (in shares)	11,014,975		11,014,975		10,956,838
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)			$ 13.20	$ 0.00	$ 0.00
Forfeited (in dollars per share)			$ 13.20		$ 0.00
Outstanding (in dollars per share)	$ 13.23		$ 13.23		$ 13.20
Management
Number of options (in shares)
Granted (in shares)			348,836		10,872,090
Weighted average exercise price (in dollars per share)
Granted (in dollars per share)			$ 14.09		$ 13.20
Director
Number of options (in shares)
Granted (in shares)			0		84,748
Weighted average exercise price (in dollars per share)
Granted (in dollars per share)					$ 13.20
Management and director options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share compensation expenses	$ 3,700	$ 1,200	$ 6,500	$ 3,300
Number of options (in shares)
Outstanding (in shares)	11,014,975		11,014,975
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)	$ 13.23		$ 13.23