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Common Stock - Narrative (Detail)
$ / shares in Units, $ in Millions
 12 Months Ended
Feb. 01, 2017
USD ($)
$ / shares
shares
Nov. 29, 2016
USD ($)
$ / shares
shares
Nov. 09, 2016
USD ($)
$ / shares
shares
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2018
shares
Class of Stock [Line Items]            
Common stock split ratio     2,200      
Common stock, shares authorized (in shares)     350,000,000 350,000,000   350,000,000
Preferred stock, undesignated shares authorized (in shares)     10,000,000      
Common Stock            
Class of Stock [Line Items]            
Sale of shares of common stock (in shares) 1,500,000     1,500,000 12,577,500  
Purchase price per share (USD per share) | $ / shares $ 17.50          
Proceeds from issuance of common stock net of underwriting discounts and expenses | $ $ 24.2          
Common Stock | Initial Public Offering            
Class of Stock [Line Items]            
Sale of shares of common stock (in shares)     11,700,000      
Purchase price per share (USD per share) | $ / shares     $ 11.00      
Net proceed received from IPO after deducting underwriting discounts and commissions | $     $ 121.0      
Common Stock | Over-Allotment Option            
Class of Stock [Line Items]            
Purchase price per share (USD per share) | $ / shares   $ 11.00        
Additional common stock, shares issued to underwriters (in shares)   878,000        
Net proceed received from underwriter option exercise after deducting underwriting discounts and commissions | $   $ 9.7