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Equity - Additional Information (Details) - USD ($)
1 Months Ended 12 Months Ended
Feb. 19, 2019
Jan. 31, 2018
Jul. 31, 2017
Jul. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Stockholders Equity [Line Items]              
Net proceeds from sale of common stock         $ 270,969,000    
Common Stock Warrants Expiring on 2020              
Stockholders Equity [Line Items]              
Common stock warrants outstanding         149,700 149,700  
Common stock warrants, exercise price         $ 3.01 $ 3.01  
Common stock warrants, expiration period         2020 2020  
Common Stock Warrants Expiring on 2021              
Stockholders Equity [Line Items]              
Common stock warrants outstanding         149,700 149,700  
Common stock warrants, exercise price         $ 3.01 $ 3.01  
Common stock warrants, expiration period         2021 2021  
ATM Sales Agreement              
Stockholders Equity [Line Items]              
Option to sell common stock for cash       $ 150,000,000      
Common stock shares sold           912,351 1,159,415
Net proceeds from sale of common stock           $ 67,600,000 $ 79,500,000
ATM Sales Agreement | Subsequent Event              
Stockholders Equity [Line Items]              
Common stock shares sold 379,707            
Net proceeds from sale of common stock $ 19,300,000            
Additional ATM Sales Agreement              
Stockholders Equity [Line Items]              
Common stock shares sold         640,257 1,338,866  
Net proceeds from sale of common stock         $ 38,100,000 $ 64,300,000  
Additional ATM Sales Agreement | Maximum              
Stockholders Equity [Line Items]              
Option to sell common stock for cash     $ 150,000,000        
Underwritten Public Offering              
Stockholders Equity [Line Items]              
Common stock shares sold   5,043,860          
Net proceeds from sale of common stock   $ 271,000,000          
Shares purchased by underwriters   657,895          
Public offering price   $ 57.00