Share-based compensation plans - Schedule of deferred shares and restricted shares and changes during period (Details) - 2009 share award and incentive plan - Deferred shares - Class A common shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Number of shares subject to options
Outstanding (in shares)	1,271,738	1,193,775
Granted (in shares)	820,544	565,621
Vested and issued (in shares)	(351,077)	(334,958)
Forfeited, cancelled or expired (in shares)	(182,183)	(152,700)
Outstanding (in shares)	1,559,022	1,271,738
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding, weighted average exercise price (in dollars per share)	$ 0.01	$ 0.01
Granted, weighted average exercise price (in dollars per share)	0.01	0.01
Vested and issued, weighted average exercise price (in dollars per share)	0.01	0.01
Forfeited, cancelled or expired, weighted average exercise price (in dollars per share)	0.01	0.01
Outstanding, weighted average exercise price (in dollars per share)	$ 0.01	$ 0.01
Aggregate intrinsic value
Outstanding, aggregate intrinsic value	$ 39,007