Annual report pursuant to Section 13 and 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
Nov. 30, 2017
Aug. 31, 2017
May 31, 2017
Apr. 30, 2017
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Class Of Stock [Line Items]                  
Common stock, shares outstanding           8,142,120 8,074,197 7,990,000 7,768,000
Issuance of Class A common Stock upon conversion of Class B units to Class A common stock 10,900 24,119 2,000   26,008        
Issuance of Class B shares, Shares     3,896 1,000          
Common stock conversion date Nov. 30, 2017 Aug. 31, 2017 May 31, 2017   Mar. 31, 2017        
Stock issued upon vesting of stock units     2017-05 2017-04