Share-based compensation - Schedule of management and director options activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of options (in shares)
Outstanding (in shares)	8,888,053	8,888,053	9,596,993
Forfeited (in shares)	(393,717)	0	(558,138)
Exercised (in shares)	(187,678)	0	(150,802)
Outstanding (in shares)	8,306,658	8,888,053	8,888,053
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)	$ 13.43	$ 13.43	$ 13.64
Forfeited (in dollars per share)	14.73	0	18.00
Exercised (in dollars per share)	10.00	0	10.00
Outstanding (in dollars per share)	$ 13.45	13.43	$ 13.43
Management and director options
Number of options (in shares)
Outstanding (in shares)	8,306,658
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)	$ 13.45
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	$ 10.00
Number of options (in shares)
Outstanding (in shares)	4,774,694
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)	$ 10.04