Quarterly report pursuant to Section 13 or 15(d)

Equity - Class B Common Stock (Detail)

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Equity - Class B Common Stock (Detail) - Class B Common Stock - shares
1 Months Ended 9 Months Ended
Sep. 30, 2018
Aug. 31, 2018
Jun. 30, 2018
May 31, 2018
Apr. 30, 2018
Mar. 31, 2018
Jan. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Class Of Stock [Line Items]                      
Common stock conversion date 2018-09 2018-08   2018-05 2018-04 2018-03 2018-01        
Stock issued upon vesting of restricted stock units, Transaction date   2018-08   2018-05              
Common stock issuance date     2018-06                
Common stock, shares outstanding 5,106,564             5,106,564 5,070,000 5,059,319 4,866,000
Cancellation of Class B common stock upon conversion of Class B units to Class A common stock (17,666) (22,285)   (24,732) (20,000)     202,000 52,000    
Issuance of Class B common stock   241,627 6,289 948       6,000 13,000    
Resale And Registration Rights Agreement                      
Class Of Stock [Line Items]                      
Cancellation of Class B 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)