Share-based compensation - Schedule of management and director options outstanding (Details) (USD $)				9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2013 Management and director options	Sep. 30, 2013 Management and director options $10.00-$10.89	Sep. 30, 2013 Management and director options $16.00-$16.89	Sep. 30, 2013 Management and director options $20.00-$20.89
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)					$ 10.00	$ 16.00	$ 20.00
Exercise price, upper limit (in dollars per share)					$ 10.89	$ 16.89	$ 20.89
Options outstanding (in shares)	11,014,975.000	10,956,838.000	0	11,014,975.000	6,608,987	2,202,994	2,202,994
Options outstanding, weighted average exercise price (in dollars per share)	$ 13.23	$ 13.20	$ 0.00	$ 13.23	$ 10.03	$ 16.03	$ 20.03
Remaining contractual life				8 years 3 months 22 days	8 years 3 months 22 days	8 years 3 months 22 days	8 years 3 months 22 days
Options exercisable (in shares)				2,133,226.000	1,279,936	426,645	426,645
Options exercisable, weighted average exercise price (in dollars per share)				$ 13.20	$ 10.00	$ 16.00	$ 20.00