Share-Based Compensation Plans - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) Employee $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional employees to participate in plan | Employee				2
Shares held in employee stock option plan, allocated (in shares) | shares		82.1918
Shares issued and outstanding to LLC (in shares) | shares		60
Deferred equity unit share (in shares) | shares		0.0001
Weighted-average grant date intrinsic value of share awards (in usd per share) | $ / shares		$ 149.45	$ 148.69	$ 140.04
Non-cash compensation expense		$ 50	$ 62	$ 70
Free cash flow compensation expense		15	19	30
Dividend expense related to equity units		7	27	2
Unrecognized compensation costs		$ 16	$ 18	$ 34
Total compensation of unvested awards expected to be recognized, weighted average period		1 year
Deferred Compensation Share-based Arrangements, Liability, Current and Noncurrent		$ 211
Deferred Compensation Share-based Arrangements, Liability, Current		$ 108
Matching Equity Units Vesting Years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share awards vesting period		2 years
Deferred Equity Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares held in employee stock option plan, allocated (in shares) | shares		41.0959
Awards settled (in shares) | shares	4,395.54
Cash paid to settle awards	$ 1
Weighted-average grant date intrinsic value of share awards (in usd per share) | $ / shares		$ 0.00	$ 0.00
Fair value of deferred equity units (in usd per share) | $ / shares		$ 367.96	304.22	$ 241.75
Deferred Equity Units | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share awards vesting period		1 year
Deferred Equity Units | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share awards vesting period		7 years
Fractional Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards settled (in shares) | shares	86,496.04
Cash paid to settle awards	$ 26
Matching Equity Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares held in employee stock option plan, allocated (in shares) | shares		41.0959
Awards settled (in shares) | shares	4,553.50
Cash paid to settle awards	$ 1
Weighted-average grant date intrinsic value of share awards (in usd per share) | $ / shares		$ 0.00	$ 0.00