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
Aug. 31, 2017
Jun. 30, 2017
May 31, 2017
Mar. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Class Of Stock [Line Items]                
Common stock conversion date 2017-08   2017-05 2017-03        
Stock issued upon vesting of stock units 2017-08   2017-05          
Common stock issuance date   2017-06            
Common stock, shares outstanding         5,070,219 4,891,000 4,866,303 4,695,000
Cancellation of Class B common stock upon conversion of Class B units to Class A common stock         52,000 59,000    
Class B common stock issued 241,628 13,468 947   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 (24,119)   (2,000) (26,008)