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Stockholders' Equity (Details Textual)
 1 Months Ended 12 Months Ended
May 08, 2015
shares
May 22, 2014
shares
May 14, 2010
shares
Jun. 30, 2016
USD ($)
May 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
plan
vote
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
May 13, 2016
shares
Preferred Stock [Abstract]                  
Preferred stock, par value ($ per share) | $ / shares           $ 0.001 $ 0.001    
Convertible preferred stock, shares issued upon conversion, conversion price ($ per share) | $ / shares           27.20      
Daily market price triggering mandatory conversion ($ per share) | $ / shares           $ 72.00      
Stockholders' Equity (Textual) [Abstract]                  
Authorized amount | $       $ 50,000,000.0          
Agent fee       2.00%          
Issuance of common stock for cash, net of financing costs (shares)         10,925,000 658,748      
Offering price ($ per share) | $ / shares         $ 8.00 $ 9.75      
Proceeds from issuance of common stock | $           $ 6,295,761 $ 81,902,363 $ 59,209,182  
Proceeds from offering | $         $ 81,900,000        
Number of active stock plans | plan           2      
Vesting period of Incentive Plan           3 years      
Contractual year term of Incentive Plan           10 years      
Number of options outstanding to purchase common stock           6,806,183 5,862,364    
Share-based compensation | $           $ 10,479,135 $ 6,186,848 $ 5,428,946  
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized | $           $ 5,800,000 $ 5,400,000    
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition           1 year 10 months 15 days 1 year 10 months 15 days    
Aggregate intrinsic value of options outstanding | $           $ 8,300,000      
Aggregate intrinsic value for options exercisable | $           $ 8,200,000      
Options exercisable, remaining contractual term           5 years 3 months 12 days      
Number of options expected to vest           1,973,141      
Options, expirations in period, weighted average exercise price ($ per share) | $ / shares           $ 9.01 $ 10.89 $ 15.95  
Options, expirations in period           69,570 83,696 70,517  
Options, grants in period, weighted average grant date fair value ($ per share) | $ / shares           $ 4.59 $ 4.60 $ 9.19  
Proceeds from stock options exercised | $           $ 1,800,000 $ 552,000 $ 1,400,000  
Options, exercises in period, aggregate intrinsic value | $           3,500,000 456,000 2,000,000  
Employee                  
Stockholders' Equity (Textual) [Abstract]                  
Share-based compensation | $           10,200,000 5,800,000 4,800,000  
Research and Development Expense                  
Stockholders' Equity (Textual) [Abstract]                  
Share-based compensation | $           4,800,000 3,200,000 2,800,000  
General and Administrative Expense                  
Stockholders' Equity (Textual) [Abstract]                  
Share-based compensation | $           $ 5,400,000 2,600,000 2,000,000  
Non Employee                  
Stockholders' Equity (Textual) [Abstract]                  
Number of options outstanding to purchase common stock           796,810      
Allocated share-based compensation expense | $           $ 321,000 385,000 $ 585,000  
Stock and Cash Based Incentive Plan                  
Stockholders' Equity (Textual) [Abstract]                  
Increased number of securities available for future issuance 2,000,000 1,250,000 500,000            
Number of potential shares authorized for issuance under a share-based compensation plan     513,833            
Reserved number of shares under the Incentive Plan           7,770,497      
Increased aggregate number of shares available for grants under the Incentive Plan           224,507      
Number of shares of vested restricted stock outstanding under the plan           798,834      
Number of common stock shares outstanding under the Incentive Plan           5,978,342      
Amended 2000 Stock Option Plan                  
Stockholders' Equity (Textual) [Abstract]                  
Number of options outstanding to purchase common stock           100,002      
Restricted Stock Units (RSUs)                  
Stockholders' Equity (Textual) [Abstract]                  
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized | $           $ 4,000,000 $ 1,200,000    
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition           2 years 2 years 1 month 6 days    
Aggregate intrinsic value of unvested | $           $ 5,500,000      
Aggregate intrinsic value of vested | $           $ 781,000      
Expected to vest (shares)           798,834 230,000    
Weighted average grant date fair value ($ per share) | $ / shares           $ 7.41 $ 7.76    
VGX Equity Compensation Plan                  
Stockholders' Equity (Textual) [Abstract]                  
Number of options outstanding to purchase common stock           727,839      
Over-Allotment Option                  
Stockholders' Equity (Textual) [Abstract]                  
Issuance of common stock for cash, net of financing costs (shares)         1,425,000        
Minimum                  
Preferred Stock [Abstract]                  
Average daily trading volume triggering mandatory conversion (shares)           6,250      
Series C Preferred Stock                  
Preferred Stock [Abstract]                  
Preferred stock, redemption price ($ per share) | $ / shares           $ 10,000      
Preferred stock, par value ($ per share) | $ / shares           $ 0.001      
Number of votes per share | vote           368      
Notice period for mandatory conversion of preferred stock           30 days      
Convertible preferred stock, shares issued upon conversion (shares)           8,456      
Warrants Assumed in June Two Thousand Nine Merger                  
Stockholders' Equity (Textual) [Abstract]                  
Stock issued during period, shares, warrants exercised           276,813 426,625    
Net proceeds from the transaction | $           $ 0 $ 2,000,000    
2016 Incentive Plan                  
Stockholders' Equity (Textual) [Abstract]                  
Number of potential shares authorized for issuance under a share-based compensation plan                 2,000,000
Reserved number of shares under the Incentive Plan                 6,000,000