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Common Stock - Additional Information (Detail) - USD ($)
6 Months Ended
Nov. 03, 2017
Jun. 30, 2019
Dec. 31, 2018
Class Of Stock [Line Items]      
Common stock, shares outstanding   65,154,729 62,862,000
Net cash proceeds received   $ 13,949,000  
2017 ATM Agreement      
Class Of Stock [Line Items]      
Issuance of common stock (in shares)   1,386,906  
Net cash proceeds received   $ 13,900,000  
Remaining proceeds from common stock, under sales agreement   $ 118,600,000  
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%