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Common Stock - Additional Information (Detail) - USD ($)
 1 Months Ended 2 Months Ended 12 Months Ended
Nov. 03, 2017
Aug. 31, 2017
Feb. 22, 2019
Dec. 31, 2018
Dec. 31, 2017
Class Of Stock [Line Items]          
Common stock, shares outstanding       62,862,478 61,533,000
Net cash proceeds received         $ 185,936,000
Common Stock          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)         22,588,000
2017 ATM Agreement          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)         840,774
Net cash proceeds received         $ 16,900,000
Remaining proceeds from common stock, under sales agreement       $ 132,800,000  
2017 ATM Agreement | Subsequent Event          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)     1,078,901    
Net cash proceeds received     $ 11,500,000    
Underwritten Public Offering | Common Stock          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)   5,750,000      
Public offering net price per share   $ 15.00      
Net proceeds from offering   $ 80,800,000      
Over-Allotment Option | Common Stock          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)   750,000      
2015 ATM Agreement          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)         15,997,202
Net cash proceeds received         $ 88,200,000
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%