Share-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period of options issued under EIP	10 years
Vesting period from the date of grant	3 years
Total intrinsic value from options exercised	$ 146	$ 107	$ 249
Tax benefit from exercise of stock options	31	$ 23	$ 55
Unrecognized compensation cost	$ 22
Total unrecognized compensation cost related to non-vested options expected to be recognized over a weighted average period (in years)	5 months 12 days
Outstanding shares (in shares)	5,786,549	5,714,658
Weighted average remaining contractual term	6 years 10 months 13 days
Risk-free rate of return	1.60%	2.90%	2.00%
Expected term (in years)	4 years 10 days	3 years 11 months 23 days	4 years
Expected volatility	18.70%	20.20%	18.30%
Expected dividend yield	0.70%	0.70%	0.70%
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period from the date of grant	3 years
Outstanding shares (in shares)			0
Total grant date fair value of shares vested and earned	$ 284	$ 228	$ 183
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 381
Weighted average grant date fair value of RSUs (in dollars per share)	$ 183.61	$ 137.38	$ 111.11
Weighted average remaining contractual term	9 months 29 days
Performance-based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period from the date of grant	3 years	3 years	3 years
Unrecognized compensation cost	$ 20
Weighted average grant date fair value of RSUs (in dollars per share)	$ 211.08	$ 153.42	$ 120.11
Total grant date fair value of shares vested and earned	$ 65	$ 41	$ 31
Weighted average remaining contractual term	8 months 23 days
Risk-free rate of return	1.60%
Expected term (in years)	1 year 10 months 24 days
Expected volatility	20.90%
Expected dividend yield	0.70%
2007 Equity Incentive Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of Class A Common Stock authorized for issuance under EIP or ESPP (in shares)	236,000,000
Share-based compensation expense under EIP	$ 393	388	312
Tax benefit under EIP	$ 63	$ 59	$ 53
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of Class A Common Stock authorized for issuance under EIP or ESPP (in shares)	20,000,000
ESPP discount from market price	15.00%