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Share-based compensation plans - Schedule of status of awards other than options, and changes during the period (Details) - 2009 share award and incentive plan - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Deferred shares | Class A common shares    
Shares    
Outstanding (in shares) 1,193,775 1,162,187
Granted (in shares) 565,621 624,261
Vested and issued (in shares) (334,958) (331,299)
Forfeited, cancelled or expired (in shares) (152,700) (261,374)
Outstanding (in shares) 1,271,738 1,193,775
Weighted average exercise price    
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 $ 15,566  
On Retirement | Restricted Stock without Performance Criteria Share-based Payments    
Shares    
Granted (in shares) 11,278