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Share capital - Additional Information (Detail) (USD $)
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Dec. 31, 2011
Oct. 07, 2011
Preferred Stock [Member]
          
Class of Stock [Line Items]          
Preferred Stock 50,000,000        
Preferred Stock par value $ 0.001        
Preferred Stock issued 0        
Class A Stock [Member]
          
Class of Stock [Line Items]          
Common stock 50,000,000        
Common stock par value $ 0.001        
Common stock issued 13,508,318 13,508,318     13,572,265
Common stock outstanding 13,508,318 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%
Class B Stock [Member]
          
Class of Stock [Line Items]          
Common stock 99,680        
Common stock par value $ 0.001        
Common stock issued 99,680 99,680      
Common stock outstanding 99,680 99,680