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Stockholders' Equity - Narrative (Details)
 1 Months Ended 12 Months Ended 18 Months Ended
Jul. 25, 2017
USD ($)
$ / shares
shares
Jun. 30, 2016
USD ($)
Dec. 31, 2017
USD ($)
day
vote
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
vote
$ / shares
shares
Jan. 01, 2018
shares
May 13, 2016
shares
Class of Stock [Line Items]                
Preferred stock, par value ($ per share) | $ / shares     $ 0.001 $ 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          
Number of trading days | day     20          
Number of consecutive trading days | day     30          
Proceeds from issuance of common stock | $     $ 94,347,922 $ 6,295,761 $ 81,902,363      
Share-based compensation | $     13,070,962 10,479,135 $ 6,186,848      
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized | $     $ 5,900,000 $ 5,800,000   $ 5,900,000    
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition     1 year 9 months 15 days 1 year 10 months 15 days        
Number of options outstanding to purchase common stock (shares) | shares     7,694,870 6,806,183   7,694,870    
Aggregate intrinsic value of options outstanding | $     $ 2,300,000          
Aggregate intrinsic value for options exercisable | $     $ 2,300,000     $ 2,300,000    
Options exercisable, remaining contractual term     5 years 10 months 15 days          
Number of options expected to vest (shares) | shares     7,694,870     7,694,870    
Options, expirations in period, weighted average exercise price ($ per share) | $ / shares     $ 9.67 $ 9.01 $ 10.89      
Options, expirations in period | shares     200,501 69,570 83,696      
Options, grants in period, weighted average grant date fair value ($ per share) | $ / shares     $ 4.33 $ 4.59 $ 4.60      
Proceeds from stock options exercised | $     $ 2,300,000 $ 1,800,000 $ 552,000      
Options, exercises in period, aggregate intrinsic value | $     519,000 3,500,000 456,000      
Employee                
Class of Stock [Line Items]                
Share-based compensation | $     12,900,000 10,200,000 5,800,000      
Research and Development Expense                
Class of Stock [Line Items]                
Share-based compensation | $     5,800,000 4,800,000 3,200,000      
General and Administrative Expense                
Class of Stock [Line Items]                
Share-based compensation | $     7,100,000 5,400,000 2,600,000      
Non Employee                
Class of Stock [Line Items]                
Allocated share-based compensation expense | $     $ 201,000 321,000 $ 385,000      
Number of options outstanding to purchase common stock (shares) | shares     701,367     701,367    
Restricted Stock Units (RSUs)                
Class of Stock [Line Items]                
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized | $     $ 5,300,000 $ 4,000,000   $ 5,300,000    
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition     1 year 9 months 2 years        
Aggregate intrinsic value of unvested | $     $ 5,100,000     $ 5,100,000    
Aggregate intrinsic value of vested | $     $ 3,600,000          
RSU's expected to vest (shares) | shares     1,234,168     1,234,168    
Weighted average grant date fair value ($ per share) | $ / shares     $ 6.66 $ 7.41 $ 7.76      
At-the-Market Exchange                
Class of Stock [Line Items]                
Proceeds from issuance of common stock | $     $ 24,200,000     $ 30,500,000    
Authorized amount | $   $ 50,000,000            
Agent fee   2.00%            
Issuance of common stock for cash, net of financing costs (shares) | shares     2,937,406     3,596,154    
Offering price ($ per share) | $ / shares     $ 8.41     $ 8.41    
Stock sales agreement, remaining authorized shares (shares) | $     $ 18,900,000     $ 18,900,000    
Minimum                
Class of Stock [Line Items]                
Average daily trading volume triggering mandatory conversion (shares) | shares     6,250          
Series C Preferred Stock                
Class of Stock [Line Items]                
Preferred stock, redemption price ($ per share) | $ / shares     $ 10,000     $ 10,000    
Preferred stock, par value ($ per share) | $ / shares     $ 0.001     $ 0.001    
Number of votes per share | vote     368     368    
Convertible preferred stock, shares issued upon conversion (shares) | shares     8,456     8,456    
Notice period for mandatory conversion of preferred stock     30 days          
Common stock | Public Offering, Not IPO                
Class of Stock [Line Items]                
Sale of stock, number of shares issued in transaction (shares) | shares 12,500,000              
Sale of stock, price per share ($ per share) | $ / shares $ 6.00              
Proceeds from issuance of common stock | $ $ 70,100,000              
2016 Incentive Plan                
Class of Stock [Line Items]                
Reserved number of shares under the Incentive Plan (shares) | shares     6,000,000     6,000,000    
Number of potential shares authorized for issuance under a share-based compensation plan (shares) | shares               2,000,000
Number of shares available for grants under the Incentive Plan (shares) | shares     3,994,511     3,994,511    
Number of shares of vested restricted stock outstanding under the plan (shares) | shares     819,507          
Number of common stock shares outstanding under the Incentive Plan (shares) | shares     1,136,057     1,136,057    
Vesting period of Incentive Plan     3 years          
Contractual year term of Incentive Plan     10 years          
2007 Incentive Plan                
Class of Stock [Line Items]                
Number of shares of vested restricted stock outstanding under the plan (shares) | shares     414,661          
Number of common stock shares outstanding under the Incentive Plan (shares) | shares     6,357,071     6,357,071    
Vesting period of Incentive Plan     3 years          
Contractual year term of Incentive Plan     10 years          
VGX Equity Compensation Plan                
Class of Stock [Line Items]                
Number of common stock shares outstanding under the Incentive Plan (shares) | shares     201,742     201,742    
Subsequent Event | 2016 Incentive Plan                
Class of Stock [Line Items]                
Increase in number of shares authorized (shares) | shares             2,000,000