Annual report pursuant to Section 13 and 15(d)

Equity - Additional Information (Detail)

v3.10.0.1
Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Dec. 21, 2016
Jun. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
May 24, 2018
May 08, 2018
Apr. 23, 2018
Schedule Of Stockholders Equity [Line Items]              
Preferred stock, shares authorized (in shares)     10,000,000 1,000,000      
Preferred stock, par value (in dollars per share)     $ 0.001 $ 0.001      
Preferred stock, shares issued (in shares)     0 0      
Preferred stock, shares outstanding (in shares)     0 0      
Common stock, shares authorized (in shares)     0 126,000,000      
Common stock, par value (in dollars per share)     $ 0.001 $ 0.001     $ 0.001
Common stock, shares issued (in shares)     0 44,987,575      
Common stock, shares outstanding (in shares)     0 41,691,541      
Treasury stock, shares (in shares)       3,296,034      
Treasury shares issued (in dollars per share)       $ 3.64      
Dividends $ 31.3            
Common stock, dividends declared (in dollars per share) $ 0.754            
Class B Common Stock              
Schedule Of Stockholders Equity [Line Items]              
Common stock, shares authorized (in shares)     100,000,000 0     100,000,000
Common stock, par value (in dollars per share)     $ 0.001 $ 0.001      
Common stock, shares issued (in shares)     42,387,571 0 42,387,571 42,737,571  
Common stock, shares outstanding (in shares)     39,464,619 0 39,217,537 39,567,537  
2010 non-plan stock option agreement options | Class B Common Stock              
Schedule Of Stockholders Equity [Line Items]              
Treasury shares issued (in dollars per share)   $ 3.64 $ 3.64        
Shares granted (in shares)     768,984        
Exercises in period, weighted average (in dollars per share)   $ 5.70 $ 5.70