Quarterly report pursuant to Section 13 or 15(d)

Equity - Class A Common Stock (Detail)

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Equity - Class A Common Stock (Detail) - Class A Common Stock - shares
 1 Months Ended
Sep. 30, 2018
Aug. 31, 2018
May 31, 2018
Apr. 30, 2018
Mar. 31, 2018
Jan. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Class Of Stock [Line Items]                  
Common stock, shares outstanding 8,345,635           8,142,120 8,131,000 8,074,000
Issuance of Class A common stock upon conversion of Class B units to Class A common stock 17,666 22,285 24,732 20,000 85,286 31,650      
Issuance of Class A common stock upon vesting of restricted stock units     1,896            
Common stock conversion date Sep. 30, 2018 Aug. 31, 2018 May 31, 2018 Apr. 30, 2018 Mar. 31, 2018 Jan. 31, 2018      
Stock issued upon vesting of restricted stock units, Transaction date     May 31, 2018