Share-based compensation - Schedule of management and director options outstanding (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding (in shares)	11,329,911.000		10,981,075.000	10,956,838
Options outstanding, weighted average exercise price (in dollars per share)	$ 13.38		$ 13.23	$ 13.20
Remaining contractual life	7 years 10 months 24 days	8 years 1 month 6 days
Management and director options
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding (in shares)	11,329,911.000
Options outstanding, weighted average exercise price (in dollars per share)	$ 13.38
Remaining contractual life	7 years 10 months 24 days
Options exercisable (in shares)	3,725,265.000
Options exercisable, weighted average exercise price (in dollars per share)	$ 13.20
Management and director options | $10.00-$10.89
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	$ 10.00
Exercise price, upper limit (in dollars per share)	$ 10.89
Options outstanding (in shares)	6,588,647
Options outstanding, weighted average exercise price (in dollars per share)	$ 10.03
Remaining contractual life	7 years 10 months 24 days
Options exercisable (in shares)	2,235,159
Options exercisable, weighted average exercise price (in dollars per share)	$ 10.00
Management and director options | $15.05-$16.89
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	$ 15.05
Exercise price, upper limit (in dollars per share)	$ 16.89
Options outstanding (in shares)	2,405,516
Options outstanding, weighted average exercise price (in dollars per share)	$ 15.94
Remaining contractual life	7 years 10 months 24 days
Options exercisable (in shares)	745,053
Options exercisable, weighted average exercise price (in dollars per share)	$ 16.00
Management and director options | $20.00-$25.05
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	$ 20.00
Exercise price, upper limit (in dollars per share)	$ 25.05
Options outstanding (in shares)	2,335,748
Options outstanding, weighted average exercise price (in dollars per share)	$ 20.21
Remaining contractual life	7 years 10 months 24 days
Options exercisable (in shares)	745,053
Options exercisable, weighted average exercise price (in dollars per share)	$ 20.00