Quarterly report pursuant to Section 13 or 15(d)

Equity (Detail)

v3.10.0.1
Equity (Detail)
 3 Months Ended 12 Months Ended
May 24, 2018
$ / shares
shares
May 08, 2018
$ / shares
shares
Apr. 23, 2018
$ / shares
shares
Dec. 31, 2018
$ / shares
shares
Sep. 30, 2018
$ / shares
shares
Schedule Of Stockholders Equity [Line Items]          
Preferred stock, shares authorized (in shares)       10,000,000 10,000,000
Preferred stock, par value (in dollars per share) | $ / shares       $ 0.001 $ 0.001
Preferred stock, shares issued (in shares)       0 0
Preferred stock, shares outstanding (in shares)       0 0
Common stock, par value (in dollars per share) | $ / shares     $ 0.001    
Treasury stock, shares (in shares)       (2,922,952) (2,922,592)
Average cost of treasury shares (in dollars per share) | $ / shares       $ 5.34 $ 5.34
Class B Common Stock          
Schedule Of Stockholders Equity [Line Items]          
Common stock, par value (in dollars per share) | $ / shares       $ 0.001 $ 0.001
Stock split, conversion ratio     25.2    
Conversion of stock, shares issued (in shares)     41,817,537    
Common stock, shares authorized (in shares)       100,000,000 100,000,000
Number of shares issued in transaction (in shares) 350,000 2,250,000      
Common stock, shares issued (in shares)       42,387,571 42,387,571
Common stock, shares outstanding (in shares)       39,464,619 39,464,619
Class A Common Stock          
Schedule Of Stockholders Equity [Line Items]          
Common stock, par value (in dollars per share) | $ / shares       $ 0.001 $ 0.001
Conversion of stock, shares issued (in shares) 350,000 2,250,000      
Common stock, shares authorized (in shares)       400,000,000 400,000,000
Number of shares issued in transaction (in shares) 350,000 9,000,000      
Common stock, shares issued (in shares)       11,950,000 11,950,000
Common stock, shares outstanding (in shares)       11,950,000 11,950,000
Treasury Stock | Class B Common Stock          
Schedule Of Stockholders Equity [Line Items]          
Conversion of stock, shares issued (in shares)     3,170,034    
IPO | Class A Common Stock          
Schedule Of Stockholders Equity [Line Items]          
Number of shares issued in transaction (in shares) 700,000 11,250,000      
Shares issued, price per share (in dollars per share) | $ / shares $ 12.00 $ 12.00