Share-based Compensation - Summary of Option Activity (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares
Aggregate Intrinsic Value
Stock price used to calculate aggregate intrinsic value (in dollars per share)	$ 199.97
Employee Stock Option
Options
Beginning balance (in shares) | shares	5,714,658
Granted (in shares) | shares	1,247,982
Forfeited (in shares) | shares	(67,193)
Expired (in shares) | shares	0
Exercised (in shares) | shares	(1,108,898)
Ending balance (in shares) | shares	5,786,549
Options exercisable at September 30, 2020 (in shares) | shares	3,425,611
Options exercisable and expected to vest at September 30, 2020 (in shares) | shares	5,718,325
Weighted- Average Exercise Price Per Share
Beginning balance (in dollars per share)	$ 90.18
Granted (in dollars per share)	182.50
Forfeited (in dollars per share)	140.17
Expired (in dollars per share)	0
Exercised (in dollars per share)	63.53
Ending balance (in dollars per share)	114.61
Options exercisable at September 30, 2020 (in dollars per share)	87.28
Options exercisable and expected to vest at September 30, 2020 (in dollars per share)	$ 113.96
Weighted- Average Remaining Contractual Term (in years)
Outstanding at September 30, 2020	6 years 10 months 13 days
Options exercisable at September 30, 2020	5 years 9 months 14 days
Options exercisable and expected to vest at September 30, 2020	6 years 10 months 2 days
Aggregate Intrinsic Value
Outstanding at September 30, 2020 | $	$ 494
Options exercisable at September 30, 2020 | $	386
Options exercisable and expected to vest at September 30, 2020 | $	$ 492