Share-based compensation plans - Schedule of status of options, and changes during the period (Details) - Share options - Class A common shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
2004 stock option plans
Number of shares subject to options
Outstanding (in shares)	15,300	66,350
Granted (in shares)	0
Exercised (in shares)	(5,022)	(3,907)
Forfeited, cancelled or expired (in shares)	(10,278)	(47,143)
Outstanding (in shares)	0	15,300
Exercisable (in shares)	0
Weighted average exercise price
Outstanding, weighted average exercise price (in dollars per share)	$ 15.99	$ 39.63
Exercised, weighted average exercise price (in dollars per share)	5.89	5.89
Forfeited, cancelled or expired, weighted average exercise price (in dollars per share)	31.02	47.90
Outstanding, weighted average exercise price (in dollars per share)	0.00	$ 15.99
Exercisable (in dollars per share)	$ 0.00
2009 share award and incentive plan
Number of shares subject to options
Outstanding (in shares)	2,689,407	2,644,489
Granted (in shares)	0	581,300
Exercised (in shares)	(64,174)	(107,552)
Forfeited, cancelled or expired (in shares)	(365,171)	(428,830)
Outstanding (in shares)	2,260,062	2,689,407
Exercisable (in shares)	1,595,372
Weighted average exercise price
Outstanding, weighted average exercise price (in dollars per share)	$ 10.75	$ 11.17
Granted (in shares)	0.00	12.59
Exercised, weighted average exercise price (in dollars per share)	10.50	8.76
Forfeited, cancelled or expired, weighted average exercise price (in dollars per share)	11.72	11.22
Outstanding, weighted average exercise price (in dollars per share)	11.57	$ 10.75
Exercisable (in dollars per share)	$ 11.39
Weighted average remaining contractual life in years
Outstanding, weighted average remaining contractual term in years	6 years 3 months 19 days
Exercisable, weighted average remaining contractual term in years	5 years 6 months
Aggregate intrinsic value
Outstanding, intrinsic value	$ 30,418
Exercisable, intrinsic value	$ 21,799