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Share Capital - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Jun. 30, 2013
Dec. 31, 2012
Jun. 30, 2012
Dec. 31, 2011
Oct. 07, 2011
Class of Stock [Line Items]                  
Preferred stock, authorized 50,000,000   50,000,000            
Preferred stock, par value $ 0.001   $ 0.001            
Preferred stock, issued 0   0            
Class A Stock [Member]
                  
Class of Stock [Line Items]                  
Common stock, authorized 50,000,000   50,000,000            
Common stock, par value $ 0.001   $ 0.001            
Common stock, issued 13,442,759   13,442,759     13,508,318     13,572,265
Common stock, outstanding 13,442,759 13,515,012 13,442,759 13,515,012 13,496,783 13,508,318 13,489,162 13,572,265 13,572,265
Repurchase of class A common stock                 675,000
Percentage of repurchase of class A common stock                 5.00%
Repurchase and cancelled stock 119,251 15,000 130,786 114,900          
Total consideration of repurchase and cancelled stock     $ 2.3            
Repurchase and cancelled stock per share     $ 17.69            
Class B Stock [Member]
                  
Class of Stock [Line Items]                  
Common stock, authorized 99,680   99,680            
Common stock, par value $ 0.001   $ 0.001            
Common stock, issued 99,680   99,680     99,680      
Common stock, outstanding 99,680   99,680     99,680