Share-based compensation - Schedule of management and director options outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding (in shares)	10,250,586	10,990,841	10,981,075	10,956,838
Options outstanding, weighted average exercise price (in dollars per share)	$ 13.52	$ 13.41	$ 13.23	$ 13.20
Management and director options
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding (in shares)	10,250,586.000
Options outstanding, weighted average exercise price (in dollars per share)	$ 13.52
Remaining contractual life	5 years 11 months 26 days	7 years 1 month 13 days
Options exercisable (in shares)	6,971,519.000
Options exercisable, weighted average exercise price (in dollars per share)	$ 13.49
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)	5,788,392
Options outstanding, weighted average exercise price (in dollars per share)	$ 10.03
Remaining contractual life	5 years 11 months 19 days
Options exercisable (in shares)	3,946,530
Options exercisable, weighted average exercise price (in dollars per share)	$ 10.02
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)	2,265,981
Options outstanding, weighted average exercise price (in dollars per share)	$ 15.94
Remaining contractual life	6 years 21 days
Options exercisable (in shares)	1,526,448
Options exercisable, weighted average exercise price (in dollars per share)	$ 15.96
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)	2,196,213
Options outstanding, weighted average exercise price (in dollars per share)	$ 20.22
Remaining contractual life	6 years
Options exercisable (in shares)	1,498,541
Options exercisable, weighted average exercise price (in dollars per share)	$ 20.13