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Common Stock - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended
Nov. 03, 2017
May 03, 2019
Mar. 31, 2019
Dec. 31, 2018
Class Of Stock [Line Items]        
Common stock, shares outstanding     65,020,320 62,862,000
Net cash proceeds received     $ 13,623,000  
2017 ATM Agreement        
Class Of Stock [Line Items]        
Issuance of common stock (in shares)     1,343,337  
Net cash proceeds received     $ 13,600,000  
Remaining proceeds from common stock, under sales agreement     $ 118,900,000  
2017 ATM Agreement | Subsequent Event        
Class Of Stock [Line Items]        
Issuance of common stock (in shares)   43,569    
Net cash proceeds received   $ 300,000    
Maximum | 2017 ATM Agreement        
Class Of Stock [Line Items]        
Common stock sales agreement aggregate sales proceeds $ 150,000,000      
Commission on gross sales proceeds of common stock 3.00%