Annual report pursuant to Section 13 and 15(d)

Equity - Schedule of Outstanding Shares of Common Stock, Treasury Shares and Additional Paid-in Capital (Detail)

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Equity - Schedule of Outstanding Shares of Common Stock, Treasury Shares and Additional Paid-in Capital (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Beginning balance $ 152,181 $ 156,283  
Treasury stock, shares (in shares)   3,296,034  
Equity-based compensation expense 975 $ 513  
Sale of treasury stock   638  
Initial public offering of Class A common stock, net of offering costs 98,009    
Cashless option exercise (2,492)    
Ending balance 299,469 152,181  
Additional Paid-in Capital      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Beginning balance 142,385 141,872  
Equity-based compensation expense 975 513  
Sale of treasury stock (453)    
Initial public offering of Class A common stock, net of offering costs 98,000    
Cashless option exercise 1,586    
Ending balance 242,493 142,385  
Treasury Stock      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Beginning balance $ (11,983) $ (12,621)  
Treasury stock, shares (in shares) 2,922,952 3,296,034 3,485,085
Issuance of treasury shares (in shares) 126,000 189,051  
Sale of treasury stock $ 458 $ 638  
Cashless option exercise (in shares) 247,082    
Cashless option exercise $ (4,078)    
Ending balance $ (15,603) $ (11,983)  
Shares of Common Stock Outstanding      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Beginning balance outstanding (in shares) 41,691,541 41,502,490  
Beginning balance $ 45 $ 45  
Issuance of treasury shares (in shares) 126,000 189,051  
Reclassification of common stock (in shares) 41,817,541    
Ending balance outstanding (in shares) 0 41,691,541  
Ending balance $ 0 $ 45  
Class A Common Stock      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Beginning balance outstanding (in shares) 0 0  
Beginning balance $ 0 $ 0  
Initial public offering (in shares) 2,600,000    
Initial public offering of Class A common stock, net of offering costs (in shares) 9,350,000    
Initial public offering of Class A common stock, net of offering costs $ 9    
Ending balance outstanding (in shares) 11,950,000 0  
Ending balance $ 12 $ 0  
Class B Common Stock      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Beginning balance outstanding (in shares) 0 0  
Beginning balance $ 0 $ 0  
Reclassification of common stock (in shares) 41,817,537    
Initial public offering (in shares) 2,600,000    
Cashless option exercise (in shares) 247,082    
Ending balance outstanding (in shares) 39,464,619 0  
Ending balance $ 42 $ 0