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, 2017	Dec. 31, 2016
2004 stock option plans
Number of shares subject to options
Outstanding (in shares)	66,350	85,650
Granted (in shares)	0
Exercised (in shares)	(3,907)	(2,548)
Forfeited, cancelled or expired (in shares)	(47,143)	(16,752)
Outstanding (in shares)	15,300	66,350
Exercisable (in shares)	15,300
Weighted average exercise price
Outstanding, weighted average exercise price (in dollars per share)	$ 39.63	$ 36.30
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)	47.90	27.75
Outstanding, weighted average exercise price (in dollars per share)	15.99	$ 39.63
Exercisable (in dollars per share)	$ 15.99
Weighted average remaining contractual life
Outstanding, weighted average remaining contractual life	8 months 12 days
Exercisable, weighted average remaining contractual life	8 months 12 days
Aggregate intrinsic value
Outstanding, aggregate intrinsic value	$ 54
Exercisable, aggregate intrinsic value	$ 54
2009 share award and incentive plan
Number of shares subject to options
Outstanding (in shares)	2,644,489	2,702,957
Granted (in shares)	581,300	518,300
Exercised (in shares)	(107,552)	(59,833)
Forfeited, cancelled or expired (in shares)	(428,830)	(516,935)
Outstanding (in shares)	2,689,407	2,644,489
Exercisable (in shares)	1,516,388
Weighted average exercise price
Outstanding, weighted average exercise price (in dollars per share)	$ 11.17	$ 10.97
Granted, weighted average exercise price (in dollars per share)	12.59	11.61
Exercised, weighted average exercise price (in dollars per share)	8.76	9.35
Forfeited, cancelled or expired, weighted average exercise price (in dollars per share)	11.22	10.77
Outstanding, weighted average exercise price (in dollars per share)	10.75	$ 11.17
Exercisable (in dollars per share)	$ 11.34
Weighted average remaining contractual life
Outstanding, weighted average remaining contractual life	7 years 3 months 18 days
Exercisable, weighted average remaining contractual life	6 years
Aggregate intrinsic value
Outstanding, aggregate intrinsic value	$ 2,940
Exercisable, aggregate intrinsic value	$ 2,087