Quarterly report pursuant to Section 13 or 15(d)

Equity - Summary of Authorized and Outstanding Equity (Detail)

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Equity - Summary of Authorized and Outstanding Equity (Detail) - shares
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Preferred shares        
Preferred stock, shares authorized 10,000,000 10,000,000    
Preferred stock, shares outstanding 0 0    
Class A Common Stock        
Common shares        
Common stock, shares authorized 50,000,000 50,000,000    
Common stock, shares outstanding 8,345,635 8,142,120 8,131,000 8,074,000
Common stock, voting rights 1 vote per share      
Class B Common Stock        
Common shares        
Common stock, shares authorized 25,000,000 25,000,000    
Common stock, shares outstanding 5,106,564 5,059,319 5,070,000 4,866,000
Common stock, voting rights 1 vote per share