Share-based compensation - Schedule of management and director options activity (Details) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Number of options (in shares)
Outstanding (in shares)	10,250,586	10,990,841
Forfeited (in shares)	(139,534)	(306,976)
Exercised (in shares)	(104,652)	(433,279)
Outstanding (in shares)	10,006,400	10,250,586
Weighted average exercise price (in dollars per share)
Outstanding (in dollars per share)	$ 13.52	$ 13.41
Forfeited (in dollars per share)	18.00	14.36
Exercised (in dollars per share)	10.00	10.00
Outstanding (in dollars per share)	$ 13.50	$ 13.52