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Stockholders' Equity Narrative (Details)
 1 Months Ended 5 Months Ended 12 Months Ended 24 Months Ended
Mar. 09, 2020
USD ($)
Feb. 07, 2020
USD ($)
May 31, 2018
USD ($)
Jun. 05, 2018
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
day
vote
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Jun. 05, 2018
USD ($)
$ / shares
shares
Mar. 11, 2020
$ / shares
Jan. 01, 2020
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            
Warrants exercised in period (shares) | shares           284,091          
Proceeds from warrant exercises           $ 902,000          
Number of warrants (shares) | shares         0 0          
Share-based compensation         $ 10,901,320 $ 10,654,622 $ 13,070,962        
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized         $ 3,400,000 $ 5,200,000          
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition         1 year 7 months 31 days 1 year 8 months 12 days          
Number of options outstanding to purchase common stock (shares) | shares         9,265,390 8,752,677          
Aggregate intrinsic value of options outstanding         $ 918,000            
Aggregate intrinsic value for options exercisable         $ 881,000            
Options exercisable, remaining contractual term         5 years 8 months            
Number of options expected to vest (shares) | shares         9,265,390            
Options, expirations in period, weighted average exercise price ($ per share) | $ / shares         $ 6.27 $ 5.17 $ 9.67        
Options, expirations in period (shares) | shares         324,502 119,091 200,501        
Options, grants in period, weighted average grant date fair value ($ per share) | $ / shares         $ 2.19 $ 2.86 $ 4.33        
Proceeds from stock options exercised         $ 100,000 $ 1,500,000 $ 2,300,000        
Options, exercises in period, aggregate intrinsic value         25,000 910,000 519,000        
Employee                      
Class of Stock [Line Items]                      
Share-based compensation         9,800,000 10,200,000 12,900,000        
Research and Development Expense                      
Class of Stock [Line Items]                      
Share-based compensation         5,900,000 5,900,000 5,800,000        
General and Administrative Expense                      
Class of Stock [Line Items]                      
Share-based compensation         3,900,000 4,300,000 7,100,000        
Non Employee                      
Class of Stock [Line Items]                      
Allocated share-based compensation expense         $ 970,000 302,000 $ 201,000        
Number of options outstanding to purchase common stock (shares) | shares         952,500            
Restricted Stock Units (RSUs)                      
Class of Stock [Line Items]                      
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized         $ 4,300,000 $ 5,100,000          
Employee service share-based compensation, nonvested awards, total compensation cost not yet recognized, period for recognition         1 year 6 months 31 days 1 year 8 months          
Aggregate intrinsic value of unvested         $ 6,800,000            
Aggregate intrinsic value of vested         $ 3,500,000            
RSU's expected to vest (shares) | shares         2,069,936            
Weighted average grant date fair value ($ per share) | $ / shares         $ 3.09 $ 4.31 $ 6.66        
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%   3.00%            
Stock sale agreement, aggregate number of shares issued (shares) | shares         3,340,678            
Sale of stock, price per share ($ per share) | $ / shares         $ 2.78            
Stock sale agreement, aggregate proceeds from issuance of stock         $ 9,100,000            
Stock sales agreement, remaining authorized sales amount         $ 62,800,000            
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      
2016 Incentive Plan                      
Class of Stock [Line Items]                      
Reserved number of shares under the Incentive Plan (shares) | shares         16,000,000           16,000,000
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         8,214,032            
Number of shares of vested restricted stock outstanding under the plan (shares) | shares         2,069,936            
Number of common stock shares outstanding under the Incentive Plan (shares) | shares         4,497,736            
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 common stock shares outstanding under the Incentive Plan (shares) | shares         4,767,654            
Vesting period of incentive plan         3 years            
Contractual year term of incentive plan         10 years            
Subsequent Event | Common stock | Sales Agreement                      
Class of Stock [Line Items]                      
Authorized amount $ 250,000,000.0 $ 200,000,000.0                  
Sale of stock, price per share ($ per share) | $ / shares                 $ 4.92    
Subsequent Event | 2016 Incentive Plan                      
Class of Stock [Line Items]                      
Increase in number of shares authorized (shares) | shares                   2,000,000