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Stockholders' Equity - Narrative (Details)
 1 Months Ended 3 Months Ended 5 Months Ended 12 Months Ended 24 Months Ended
Jul. 25, 2017
USD ($)
$ / shares
shares
May 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 05, 2018
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
day
vote
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Jun. 05, 2018
USD ($)
$ / 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        
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         $ 29,227,776 $ 94,347,922 $ 6,295,761      
Warrants exercised in period (shares) | shares         284,091 0        
Proceeds from warrant exercises     $ 902,000   $ 902,000          
Number of warrants (shares) | shares         0 284,091        
Share-based compensation         $ 10,654,622 $ 13,070,962 $ 10,479,135      
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized         $ 5,200,000 $ 5,900,000        
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition         1 year 7 months 31 days 1 year 9 months 15 days        
Number of options outstanding to purchase common stock (shares) | shares         8,752,677 7,694,870        
Aggregate intrinsic value of options outstanding         $ 1,600,000          
Aggregate intrinsic value for options exercisable         $ 1,600,000          
Options exercisable, remaining contractual term         5 years 9 months          
Number of options expected to vest (shares) | shares         8,752,677          
Options, expirations in period, weighted average exercise price ($ per share) | $ / shares         $ 5.17 $ 9.67 $ 9.01      
Options, expirations in period | shares         119,091 200,501 69,570      
Options, grants in period, weighted average grant date fair value ($ per share) | $ / shares         $ 2.86 $ 4.33 $ 4.59      
Proceeds from stock options exercised         $ 1,500,000 $ 2,300,000 $ 1,800,000      
Options, exercises in period, aggregate intrinsic value         910,000 519,000 3,500,000      
Employee                    
Class of Stock [Line Items]                    
Share-based compensation         10,200,000 12,900,000 10,200,000      
Research and Development Expense                    
Class of Stock [Line Items]                    
Share-based compensation         5,900,000 5,800,000 4,800,000      
General and Administrative Expense                    
Class of Stock [Line Items]                    
Share-based compensation         4,300,000 7,100,000 5,400,000      
Non Employee                    
Class of Stock [Line Items]                    
Allocated share-based compensation expense         $ 302,000 201,000 $ 321,000      
Number of options outstanding to purchase common stock (shares) | shares         628,125          
Restricted Stock Units (RSUs)                    
Class of Stock [Line Items]                    
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized         $ 5,100,000 $ 5,300,000        
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition         1 year 7 months 31 days 1 year 9 months        
Aggregate intrinsic value of unvested         $ 6,800,000          
Aggregate intrinsic value of vested         $ 2,400,000          
RSU's expected to vest (shares) | shares         1,688,017          
Weighted average grant date fair value ($ per share) | $ / shares         $ 4.31 $ 6.66 $ 7.41      
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          
Preferred stock, par value ($ per share) | $ / shares         $ 0.001          
Number of votes per share | vote         368          
Convertible preferred stock, shares issued upon conversion (shares) | shares         8,456          
Notice period for mandatory conversion of preferred stock         30 days          
Common stock | Sales Agreement                    
Class of Stock [Line Items]                    
Authorized amount   $ 100,000,000.0                
Agent fee   3.00%                
Stock sale agreement, aggregate number of shares issued (shares) | shares         5,354,075          
Sale of stock, price per share ($ per share) | $ / shares         $ 5.27          
Stock sale agreement, aggregate proceeds from issuance of stock         $ 27,700,000          
Stock sales agreement, remaining authorized sales amount         71,800,000          
Proceeds from issuance of common stock         $ 29,200,000          
Common stock | ATM Sales Agreement                    
Class of Stock [Line Items]                    
Stock sale agreement, aggregate number of shares issued (shares) | shares       314,950   2,937,406   3,911,104    
Sale of stock, price per share ($ per share) | $ / shares       $ 5.07   $ 8.41   $ 5.07    
Stock sale agreement, aggregate proceeds from issuance of stock       $ 1,600,000   $ 24,200,000   $ 32,100,000    
Common stock | Public Offering, Not IPO                    
Class of Stock [Line Items]                    
Sale of stock, price per share ($ per share) | $ / shares $ 6.00                  
Sale of stock, number of shares issued in transaction (shares) | shares 12,500,000                  
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         8,000,000         6,000,000
Number of potential shares authorized for issuance under a share-based compensation plan (shares) | shares                   2,000,000
Increase in number of shares authorized (shares) | shares                 2,000,000  
Number of shares available for grants under the Incentive Plan (shares) | shares         3,039,566          
Number of shares of vested restricted stock outstanding under the plan (shares) | shares         1,519,187          
Number of common stock shares outstanding under the Incentive Plan (shares) | shares         3,062,183          
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         168,830          
Number of common stock shares outstanding under the Incentive Plan (shares) | shares         5,690,494          
Vesting period of Incentive Plan         3 years          
Contractual year term of Incentive Plan         10 years