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Common Stock - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
$ / shares
shares
Feb. 01, 2017
USD ($)
$ / shares
shares
Nov. 29, 2016
USD ($)
$ / shares
shares
Nov. 09, 2016
USD ($)
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Dec. 31, 2016
shares
Class Of Stock [Line Items]            
Common stock split ratio       2,200    
Common stock, shares authorized 350,000,000     350,000,000 350,000,000 350,000,000
Preferred stock, undesignated shares authorized       10,000,000    
Common Stock            
Class Of Stock [Line Items]            
Sale of shares of common stock   1,500,000     1,500,000 12,577,500
Purchase price per share | $ / shares $ 17.50 $ 17.50     $ 17.50  
Proceeds from issuance of common stock | $     $ 0      
Proceeds from issuance of common stock net of underwriting discounts and expenses | $   $ 24,200,000        
Stock sold by selling shareholders 4,450,000          
Underwriters option period 30 days          
Common Stock | Maximum            
Class Of Stock [Line Items]            
Additional common stock shares granted by selling shareholders to underwriters option 893       893  
Common Stock | Initial Public Offering            
Class Of Stock [Line Items]            
Sale of shares of common stock       11,700,000    
Purchase price per share | $ / shares       $ 11.00    
Net proceed received from IPO after deducting underwriting discounts and commissions | $       $ 121,000,000    
Common Stock | Over-Allotment Option            
Class Of Stock [Line Items]            
Purchase price per share | $ / shares     $ 11.00      
Additional common stock, shares issued to underwriters     878,000      
Net proceed received from underwriter option exercise after deducting underwriting discounts and commissions | $     $ 9,700,000