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
May 31, 2018
Apr. 30, 2018
Mar. 31, 2018
Jan. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Class Of Stock [Line Items]                
Common stock, shares outstanding         8,305,684 8,142,120 8,107,000 8,074,000
Issuance of Class A common stock upon conversion of Class B units to Class A common stock 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 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