Quarterly report pursuant to Section 13 or 15(d)

Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail)

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Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail) - $ / shares
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2016
Dec. 31, 2017
Sep. 30, 2017
Class Of Stock [Line Items]        
Preferred stock, par value $ 0.01   $ 0.01  
Class A Common Stock        
Class Of Stock [Line Items]        
Common stock, par value $ 0.01   $ 0.01  
Common stock, voting rights 1 vote per share      
Common stockholders rights percentage 100.00%      
Restricted stock units granted   10,582    
Restricted stock units unvested 3,792      
Common stock, shares outstanding 8,345,635 8,074,000 8,142,120 8,131,000
Class B Common Stock        
Class Of Stock [Line Items]        
Common stock, par value $ 0.01   $ 0.01  
Common stock, voting rights 1 vote per share      
Restricted stock units granted 243,523      
Class of share exchangeable to another class One share of Class A common stock      
Common stock, shares outstanding 5,106,564 4,866,000 5,059,319 5,070,000