Share-based Compensation - Summary of RSU Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Aggregate Intrinsic Value
Stock price used to calculate aggregate intrinsic value (in dollars per share)	$ 172.01
Restricted Stock Units (RSUs)
Restricted stock
Beginning balance (in shares)	5,204,454
Granted (in shares)	2,785,534
Vested (in shares)	(2,450,257)
Forfeited (in shares)	(372,972)
Ending balance (in shares)	5,166,759	5,204,454
Weighted-Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 96.50
Granted (in dollars per share)	137.38	$ 111.11	$ 81.67
Vested (in dollars per share)	93.12
Forfeited (in dollars per share)	115.15
Ending balance (in dollars per share)	$ 118.79	$ 96.50
Weighted-Average Remaining Contactual Term
Outstanding at September 30, 2019	25 days
Aggregate Intrinsic Value
Outstanding at September 30, 2019	$ 889