v3.21.2
Share-based Compensation - Summary of Performance-based Shares Activity (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Aggregate Intrinsic Value      
Stock price used to calculate aggregate intrinsic value (in dollars per share) $ 222.75    
Performance-based Shares      
Shares      
Beginning balance (in shares) 994,800    
Granted (in shares) 432,714    
Vested and earned (in shares) (359,894)    
Unearned (in shares) (203,760)    
Forfeited (in shares) 0    
Ending balance (in shares) 863,860 994,800  
Weighted- Average Grant Date Fair Value      
Beginning balance (in dollars per share) $ 171.33    
Granted (in dollars per share) 229.81 $ 211.08 $ 153.42
Vested and earned (in dollars per share) 130.98    
Unearned (in dollars per share) 224.79    
Forfeited (in dollars per share) 0    
Ending balance (in dollars per share) $ 204.82 $ 171.33  
Weighted- Average Remaining Contractual Term (in years)      
Outstanding at September 30, 2021 9 months 25 days    
Aggregate Intrinsic Value      
Outstanding at September 30, 2021 $ 192