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
 6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2016
Mar. 31, 2019
Dec. 31, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 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 1,896            
Common stock, shares outstanding 8,623,720   8,535,000 8,518,096 8,306,000 8,259,000 8,142,000
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 276,963            
Class of share exchangeable to another class One share of Class A common stock            
Common stock, shares outstanding 4,832,839   4,919,000 4,934,103 4,905,000 4,942,000 5,059,000