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Stockholders' Equity - Incentive Plans (Detail) - USD ($)
6 Months Ended
Jan. 01, 2018
Jan. 09, 2014
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Maximum          
Incentive plans          
Expiration period     10 years    
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     $ 512,313 $ 103,638  
Cash proceeds from exercise of stock options     $ 37,600 27,358  
Options exercised     30,362    
Fair value of shares vested       1,573  
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     $ 10,995,118    
Period for unrecognized compensation expense related to unvested options yet has not been recognized     3 years    
Intrinsic value of options exercised     $ 721,337 82,249  
Cash proceeds from exercise of stock options     $ 353,850 70,204  
Options exercised     54,624    
Fair value of shares vested     $ 2,204,404 $ 2,252,276  
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 | 2013 Equity Incentive Plan          
Authorized shares          
Common stock authorized under 2013 plan 3,867,994 1,000,000      
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