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
 9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2016
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 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 9,180,013   8,624,000 8,535,000 8,518,096 8,346,000 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 35,336                
Class of share exchangeable to another class One share of Class A common stock                
Common stock, shares outstanding 5,180,883   4,833,000 4,919,000 4,934,103 5,107,000 4,905,000 4,942,000 5,059,000