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Share-Based Compensation - Additional Information (Detail) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended 60 Months Ended
Sep. 11, 2018
Aug. 14, 2018
Oct. 17, 2017
Aug. 31, 2018
Jul. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2014
Sep. 30, 2018
Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
IPO issuance, shares 1,344,500                    
Unrecognized compensation cost related to unvested share-based compensation arrangements           $ 8.8       $ 8.8  
Unrecognized compensation cost, period for recognition           1 year 6 months          
Share-based compensation expense           $ 12.9 $ 2.3 $ 4.7      
IPO [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
IPO issuance, shares   9,630,000                  
Stock Appreciation Rights [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Number of shares vested           622,238 1,460,812 776,667   3,687,218  
Number of shares authorized                 5,000,000    
Number of shares cancelled           517,775          
Number of shares unvested           0 1,140,013 2,223,333   0 2,372,500
Number of shares granted           0 384,160 827,500   4,204,993  
Shares expiration period                 10 years    
Shares vesting period                 3 years    
Number of shares exercised                   2,088,333  
Restricted Stock Units [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Number of shares vested                   44,623  
Number of shares authorized     1,250,000                
Number of shares granted                   536,538  
Shares vesting period     3 years                
2018 Equity Incentive Plan [Member] | Restricted Stock [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Number of shares vested         998,522 32,500 0        
Number of shares authorized         2,500,000            
IPO issuance, shares         2,249,147            
Number of shares cancelled       966,022 491,915 0          
Unvested restricted shares issued         491,198            
Number of shares unvested           1,250,625 0 0   1,250,625  
Number of shares granted   1,266,034     2,249,147   0        
2018 Equity Incentive Plan [Member] | Stock Appreciation Rights [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Number of shares vested       1,598,885 966,022            
IPO issuance, shares         1,266,034            
Number of shares cancelled       300,012              
Number of shares unvested         300,012            
2018 Equity Incentive Plan [Member] | Stock Appreciation Rights [Member] | IPO [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Unvested restricted shares issued       517,775              
2018 Equity Incentive Plan [Member] | Restricted Stock Units [Member] | IPO [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Number of shares cancelled       491,915              
2011 Plan [Member] | Restricted Stock [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Number of shares vested         1,978,550            
Number of shares cancelled         470,355            
Number of shares granted         2,448,905            
2017 Plan [Member] | Restricted Stock [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Number of shares vested         10,412            
Number of shares cancelled         20,843            
Number of shares granted         31,255