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Summary of Information Regarding Option Activity (Detail) (USD $)
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Options Outstanding    
Options outstanding at January 1, 2013 2,315,090 2,622,400
Granted 325,000  
Exercised (310,300)  
Forfeited/Cancelled (98,200)  
Options outstanding at March 31, 2013 2,231,590 2,622,400
Options exercisable at March 31, 2013 1,223,100  
Vested and expected to vest at March 31, 2013 2,121,159 [1]  
Weighted average exercise price per share    
Options outstanding at January 1, 2013 $ 4.20 $ 4.17
Granted $ 6.23  
Exercised $ 4.86  
Forfeited/Cancelled $ 4.32  
Options outstanding at March 31, 2013 $ 4.40 $ 4.17
Options exercisable at March 31, 2013 $ 4.28  
Vested and expected to vest at March 31, 2013 $ 4.37 [1]  
Weighted average remaining contractual term (in years)    
Options outstanding at March 31, 2013 6 years 8 months 12 days  
Options exercisable at March 31, 2013 5 years 4 days  
Vested and expected to vest at March 31, 2013 6 years 7 months 17 days [1]  
Aggregate intrinsic value    
Options outstanding at March 31, 2013 $ 5,618,691  
Options exercisable at March 31, 2013 3,241,959  
Vested and expected to vest at March 31, 2013 $ 5,417,308 [1]  
[1] This represents the number of vested options as of March 31, 2013 plus the number of unvested options expected to vest as of March 31, 2013 based on the unvested outstanding options at March 31, 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.