Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail) - $ / shares
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
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    
Restricked stock units unvested 5,687      
Common stock, shares outstanding 8,131,220 8,074,197 8,049,000 7,990,000
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 486,098      
Class of share exchangeable to another class One share of Class A common stock      
Common stock, shares outstanding 5,070,219 4,866,303 4,891,000 4,695,000