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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 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
Preferred Stock [Member]
Dec. 31, 2012
Class A Stock [Member]
Dec. 31, 2011
Class A Stock [Member]
Dec. 31, 2010
Class A Stock [Member]
Oct. 07, 2011
Class A Stock [Member]
Dec. 31, 2009
Class A Stock [Member]
Dec. 31, 2012
Class B Stock [Member]
Dec. 31, 2011
Class B Stock [Member]
Dec. 31, 2010
Class B Stock [Member]
Class of Stock [Line Items]                                      
Preferred stock                     50,000,000                
Preferred stock par value                     $ 0.001                
Preferred stock issued                     0                
Common stock                       50,000,000         99,680    
Common stock par value                       $ 0.001         $ 0.001    
Common stock issued                       13,508,318 13,572,265   13,572,265   99,680 99,680  
Common stock outstanding                       13,508,318 13,572,265 13,268,522   13,118,001 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                       121,599              
Total Consideration of repurchase and cancelled stock                       $ 1.9              
Repurchase and cancelled stock per share                       $ 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