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, 2021
Dec. 31, 2020
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
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 13,462 8,242            
Restricted stock units unvested 21,704              
Common stock, shares outstanding 9,653,214 9,650,692 9,670,000 9,658,000 9,545,000 9,520,000 9,506,000 9,330,000
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 170,854              
Class of share exchangeable to another class one-for-one basis              
Common stock, shares outstanding 4,815,460 4,721,646 4,782,000 4,770,000 4,828,000 4,852,000 4,855,000 5,031,000