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Share Capital - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Class A Stock [Member]
Dec. 31, 2012
Class A Stock [Member]
Dec. 31, 2011
Class A Stock [Member]
Oct. 07, 2011
Class A Stock [Member]
Dec. 31, 2010
Class A Stock [Member]
Dec. 31, 2013
Class B Stock [Member]
Dec. 31, 2012
Class B Stock [Member]
Dec. 31, 2011
Class B Stock [Member]
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                  
Common stock, authorized                     50,000,000         99,680    
Common stock, par value                     $ 0.001         $ 0.001    
Common stock, issued                     13,377,967 13,508,318   13,572,265   99,680 99,680  
Common stock, outstanding                     13,377,967 13,508,318 13,572,265 13,572,265 13,268,522 99,680 99,680  
Repurchase of class A common stock                           675,000        
Percentage of repurchase of class A common stock                           5.00%        
Repurchase and cancelled stock                     200,578 121,599             
Total consideration of repurchase and cancelled stock                     $ 3.6 $ 1.9            
Repurchase and cancelled stock per share                     $ 18.07 $ 15.35            
Dividends per share $ 0.11 $ 0.11 $ 0.11 $ 0.11 $ 0.11 $ 0.11 $ 0.11 $ 0.11 $ 0.44 $ 0.44 $ 0.44 $ 0.44 $ 0.44     $ 0.44 $ 0.44 $ 0.44