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Share-based Compensation - Summary of RSU Activity (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Aggregate Intrinsic Value      
Stock price used to calculate aggregate intrinsic value (in dollars per share) $ 199.97    
Restricted Stock Units (RSUs)      
Restricted stock      
Beginning balance (in shares) 5,166,759    
Granted (in shares) 2,352,714    
Vested (in shares) (2,561,379)    
Forfeited (in shares) (267,594)    
Ending balance (in shares) 4,690,500 5,166,759  
Weighted-Average Grant Date Fair Value      
Beginning balance (in dollars per share) $ 118.79    
Granted (in dollars per share) 183.61 $ 137.38 $ 111.11
Vested (in dollars per share) 110.73    
Forfeited (in dollars per share) 147.70    
Ending balance (in dollars per share) $ 154.06 $ 118.79  
Weighted-Average Remaining Contactual Term      
Outstanding at September 30, 2020 9 months 29 days    
Aggregate Intrinsic Value      
Outstanding at September 30, 2020 $ 938