Annual report pursuant to Section 13 and 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
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
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%      
Common stock, shares outstanding 8,142,120 8,074,197 7,990,000 7,768,000
Restricted stock units granted   10,582    
Restricked stock units unvested 5,687      
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      
Class of share exchangeable to another class One share of Class A common stock      
Common stock, shares outstanding 5,059,319 4,866,303 4,695,000 4,520,000
Restricted stock units granted 486,098