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Fair value assumptions for share-based compensation plans (Details) (Class A common shares, USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Deferred shares without performance criteria
     
Share-based compensation plans      
Expected life of awards 3 years    
Restricted shares without performance criteria
     
Share-based compensation plans      
Expected life of awards 3 years    
Deferred shares with performance criteria
     
Share-based compensation plans      
Expected life of awards 3 years    
2009 share award and incentive plan | Share options
     
Share-based compensation plans      
Expected share price volatility, minimum (as a percent) 50.00% 58.00% 56.00%
Expected share price volatility, maximum (as a percent) 60.00% 60.00% 58.00%
Risk-free interest rate, minimum (as a percent) 1.30% 1.03% 0.97%
Risk-free interest rate, maximum (as a percent) 1.74% 1.69% 1.40%
Expected annual dividends per share (in dollars per share) $ 0 $ 0 $ 0
Expected life of awards   8 years 5 years
2009 share award and incentive plan | Deferred and restricted shares without performance criteria
     
Share-based compensation plans      
Expected share price volatility, minimum (as a percent) 33.00% 52.00% 37.00%
Expected share price volatility, maximum (as a percent) 51.00% 67.00% 82.00%
Risk-free interest rate, minimum (as a percent) 0.04% 0.10% 0.13%
Risk-free interest rate, maximum (as a percent) 0.67% 0.40% 1.20%
Expected annual dividends per share (in dollars per share) $ 0 $ 0 $ 0
2009 share award and incentive plan | Deferred shares with performance criteria
     
Share-based compensation plans      
Expected share price volatility (as a percent) 52.00% 59.00% 88.00%
Risk-free interest rate (as a percent)   0.47%  
Risk-free interest rate, minimum (as a percent) 0.26%   0.97%
Risk-free interest rate, maximum (as a percent) 0.39%   1.40%
Expected annual dividends per share (in dollars per share) $ 0 $ 0 $ 0
Expected life of awards   3 years 3 years
2007 stock appreciation rights plan | Stock appreciation rights
     
Share-based compensation plans      
Expected share price volatility (as a percent) 42.00% 47.00%  
Expected share price volatility, minimum (as a percent)     51.00%
Expected share price volatility, maximum (as a percent)     59.00%
Risk-free interest rate (as a percent) 0.11% 0.16%  
Risk-free interest rate, minimum (as a percent)     0.28%
Risk-free interest rate, maximum (as a percent)     0.45%
Expected annual dividends per share (in dollars per share) $ 0 $ 0 $ 0
Expected life of awards 8 months 11 months 1 year
Minimum | 2009 share award and incentive plan | Share options
     
Share-based compensation plans      
Expected life of awards 4 years 6 months    
Minimum | 2009 share award and incentive plan | Deferred and restricted shares without performance criteria
     
Share-based compensation plans      
Expected life of awards 18 days 2 months 4 months
Minimum | 2009 share award and incentive plan | Deferred shares with performance criteria
     
Share-based compensation plans      
Expected life of awards 2 years    
Maximum | 2009 share award and incentive plan | Share options
     
Share-based compensation plans      
Expected life of awards 8 years    
Maximum | 2009 share award and incentive plan | Deferred and restricted shares without performance criteria
     
Share-based compensation plans      
Expected life of awards 3 years 3 years 3 years
Maximum | 2009 share award and incentive plan | Deferred shares with performance criteria
     
Share-based compensation plans      
Expected life of awards 3 years