Share-based Compensation - Summary of Performance-based Shares 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
Performance-based Shares
Shares
Beginning balance (in shares)	999,416
Granted (in shares)	540,538
Vested (in shares)	(419,908)
Unearned (in shares)	0
Forfeited (in shares)	(49,356)
Ending balance (in shares)	1,070,690	999,416
Weighted- Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 102.07
Granted (in dollars per share)	153.42	$ 120.11	$ 86.37
Vested (in dollars per share)	97.71
Unearned (in dollars per share)	0
Forfeited (in dollars per share)	127.66
Ending balance (in dollars per share)	$ 129.08	$ 102.07
Weighted- Average Remaining Contractual Term (in years)
Outstanding at September 30, 2019	24 days
Aggregate Intrinsic Value
Outstanding at September 30, 2019	$ 184