Share-based compensation - Schedule of management and director options outstanding (Details) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding (in shares)	8,888,053	9,596,993	10,250,586
Options outstanding, weighted average exercise price (in dollars per share)	$ 13.43	$ 13.64	$ 13.52
Management and director options
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding (in shares)	8,888,053.000
Options outstanding, weighted average exercise price (in dollars per share)	$ 13.43
Remaining contractual life	4 years 5 months 4 days	4 years 11 months
Options exercisable (in shares)	8,678,752.000
Options exercisable, weighted average exercise price (in dollars per share)	$ 13.35
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)	5,123,532
Options outstanding, weighted average exercise price (in dollars per share)	$ 10.04
Remaining contractual life	4 years 4 months 1 day
Options exercisable (in shares)	5,081,671
Options exercisable, weighted average exercise price (in dollars per share)	$ 10.03
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)	1,917,145
Options outstanding, weighted average exercise price (in dollars per share)	$ 15.93
Remaining contractual life	4 years 6 months 1 day
Options exercisable (in shares)	1,819,471
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)	1,847,376
Options outstanding, weighted average exercise price (in dollars per share)	$ 20.26
Remaining contractual life	4 years 6 months 1 day
Options exercisable (in shares)	1,777,610
Options exercisable, weighted average exercise price (in dollars per share)	$ 20.17