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, 2020
Dec. 31, 2016
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
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 one vote for each share                
Common stockholders rights percentage 100.00%                
Restricted stock units granted 8,242 10,582              
Restricted stock units unvested 8,242                
Common stock, shares outstanding 9,544,607   9,520,000 9,506,000 9,329,879 9,180,000 8,624,000 8,535,000 8,518,000
Class B Common Stock                  
Class Of Stock [Line Items]                  
Common stock, par value $ 0.01       $ 0.01        
Common stock, voting rights one vote for each share                
Restricted stock units granted 74,906                
Class of share exchangeable to another class one-for-one basis                
Common stock, shares outstanding 4,827,731   4,852,000 4,855,000 5,031,017 5,181,000 4,833,000 4,919,000 4,934,000