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Shareholders' equity (Details 5) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Stock options
   
Class of Stock    
Total non-cash share-based compensation expense $ 0.5  
Stock options | Class A common shares
   
Class of Stock    
Shares forfeited (41,250)  
Awards outstanding (in shares) 2,118,750  
Restricted stock units
   
Class of Stock    
Total non-cash share-based compensation expense   $ 0.2
Shares forfeited 2,000  
Awards outstanding (in shares) 90,000