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Common Stock - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Nov. 03, 2017
Aug. 31, 2017
Dec. 31, 2017
Dec. 31, 2015
Dec. 31, 2016
Class Of Stock [Line Items]          
Common stock, shares outstanding     61,532,812   38,599,000
Net cash proceeds received     $ 185,936,000 $ 183,897,000  
Common Stock          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)     22,588,000 7,353,000  
2017 ATM Agreement          
Class Of Stock [Line Items]          
Issuance of common stock (in shares)     840,774    
Net cash proceeds received     $ 16,900,000    
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%