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Stockholders' Equity - Incentive Plans (Detail) - USD ($)
 12 Months Ended
Jan. 01, 2008
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Jan. 01, 2018
2003 Stock Incentive Plan            
Incentive plans            
Period from date of termination that any vested options shall expire   60 days        
Expiration period   10 years        
Intrinsic value of options exercised   $ 103,638 $ 97,707 $ 667,258    
Cash proceeds from exercise of stock options   $ 27,357 $ 44,771 $ 114,124    
Options exercised   16,608 28,368 99,029    
Fair value of shares vested   $ 1,573 $ 16,024 $ 50,310    
2003 Stock Incentive Plan | Upon first anniversary of start date            
Incentive plans            
Percent of shares subject to option grant that will vest   25.00%        
2003 Stock Incentive Plan | Each month after the first anniversary            
Incentive plans            
Percent of shares subject to option grant that will vest   2.083%        
2013 Equity Incentive Plan            
Incentive plans            
Period from date of termination that any vested options shall expire   90 days        
Expiration period   10 years        
Unrecognized compensation expense related to unvested options   $ 3,935,996        
Period for unrecognized compensation expense related to unvested options yet has not been recognized   2 years 2 months 12 days        
Intrinsic value of options exercised   $ 385,701 $ 2,275      
Options exercised   54,521 3,500 0 0  
Fair value of shares vested   $ 3,506,568 $ 3,053,086 $ 2,668,712    
2013 Equity Incentive Plan | Upon first anniversary of start date            
Incentive plans            
Percent of shares subject to option grant that will vest   25.00%        
2013 Equity Incentive Plan | Each month after the first anniversary            
Incentive plans            
Percent of shares subject to option grant that will vest   2.083%        
Common Stock            
Incentive plans            
Options exercised   71,129        
Common Stock | 2013 Equity Incentive Plan            
Authorized shares            
Common stock authorized under 2013 plan           3,867,994
Automatic increase in number of shares reserved for issuance as a percentage of the total common stock outstanding at the end of the prior year   3.00%       3.00%
Maximum number of shares that may be issued pursuant to exercise of incentive stock   20,000,000        
Increase in number of shares of common stock 1,030,793          
Common Stock | 2013 Equity Incentive Plan | Maximum            
Authorized shares            
Common stock authorized under 2013 plan   1,000,000