Annual report pursuant to Section 13 and 15(d)

Equity - Additional Information (Detail)

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Equity - Additional Information (Detail) - shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Equity [Line Items]    
Ownership percentage of common stock shares outstanding of chairman 85.00%  
Class B Common Stock Converted Into Class A Common Stock [Member]    
Equity [Line Items]    
Stock conversion ratio 100.00%  
Class B Common Stock [Member]    
Equity [Line Items]    
Common stock, shares outstanding 5,400,000 5,400,000
Common stock voting rights ten votes on all matters submitted to a vote of shareholders.  
Class B Common Stock [Member] | Board of Directors Chairman [Member]    
Equity [Line Items]    
Ownership percentage of common stock shares outstanding of chairman 100.00%  
Common stock, shares outstanding 5,400,000  
Class A Common Stock [Member]    
Equity [Line Items]    
Common stock, shares outstanding 9,769,961 9,752,531
Common stock voting rights one vote for each share held on all matters submitted to a vote of shareholders.  
Class A Common Stock [Member] | Board of Directors Chairman [Member]    
Equity [Line Items]    
Common stock, shares outstanding 348,682