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Summary of Movement in Non-Vested Share Options (Detail) - Employee Stock Option - $ / shares
 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Options Outstanding Number of Shares    
Beginning Balance 968,853  
Granted 256,191  
Vested (353,888)  
Forfeited (16,822)  
Ending Balance 854,334  
Weighted Average Exercise Price    
Beginning balance $ 42.14  
Granted 69.61  
Vested 37.04  
Forfeited 32.27  
Ending balance 52.69  
Weighted Average Fair Value    
Beginning Balance 13.69  
Granted 20.10 $ 19.75
Vested 12.58  
Forfeited 11.78  
Ending Balance $ 16.11