Quarterly report pursuant to Section 13 or 15(d)

Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail)

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Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2016
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Class Of Stock [Line Items]          
Preferred stock, par value $ 0.01   $ 0.01    
Class A Common Stock          
Class Of Stock [Line Items]          
Common stock, par value $ 0.01   $ 0.01    
Common stock, voting rights 1 vote per share        
Common stockholders rights percentage 100.00%        
Restricted stock units granted   10,582      
Restricted stock units unvested 3,791        
Common stock, shares outstanding 8,535,170   8,518,096 8,259,000 8,142,000
Class B Common Stock          
Class Of Stock [Line Items]          
Common stock, par value $ 0.01   $ 0.01    
Common stock, voting rights 1 vote per share        
Restricted stock units granted 243,523        
Class of share exchangeable to another class One share of Class A common stock        
Common stock, shares outstanding 4,918,545   4,934,103 4,942,000 5,059,000