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Summary of Information Regarding Option Activity (Detail) (USD $)
 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Options Outstanding    
Options outstanding at January 1, 2013 2,315,090 2,625,690
Granted 541,052  
Exercised (884,954)  
Forfeited/Cancelled (300,500)  
Options outstanding at September 30, 2013 1,670,688 2,625,690
Options exercisable at September 30, 2013 866,136  
Vested and expected to vest at September 30, 2013 1,576,516 [1]  
Weighted average exercise price per share    
Options outstanding at January 1, 2013 $ 4.20 $ 4.11
Granted $ 6.97  
Exercised $ 4.36  
Forfeited/Cancelled $ 5.03  
Options outstanding at September 30, 2013 $ 4.87 $ 4.11
Options exercisable at September 30, 2013 $ 4.19  
Vested and expected to vest at September 30, 2013 $ 4.77 [1]  
Weighted average remaining contractual term (in years)    
Options outstanding at September 30, 2013 6 years 10 months 28 days  
Options exercisable at September 30, 2013 5 years 1 month 28 days  
Vested and expected to vest at September 30, 2013 6 years 9 months 26 days [1]  
Aggregate intrinsic value    
Options outstanding at September 30, 2013 $ 10,394,325  
Options exercisable at September 30, 2013 5,972,968  
Vested and expected to vest at September 30, 2013 $ 9,970,772 [1]  
[1] This represents the number of vested options as of September 30, 2013 plus the number of unvested options expected to vest as of September 30, 2013 based on the unvested outstanding options at September 30, 2013 adjusted for estimated forfeiture rates of 8% for awards granted to non-executive level employees and 3% for awards granted to executive level employees.