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Share-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Mar. 13, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of vesting in first anniversary		30.00%
Percentage of vesting in second anniversary		30.00%
Percentage of vesting in third anniversary		40.00%
Net share-based compensation expense		$ 22	$ 23	$ 18
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price equal to market price on the date of grant		100.00%
Vesting period (in years)		3 years
Expiration period, years		10 years
Unrecognized compensation cost		$ 3
Weighted-average period, years		1 year
Options outstanding, aggregate intrinsic value		$ 23
Intrinsic value of options exercised		6	17	2
Cash received from exercises		9	20	$ 4
Tax benefits from the exercise of stock options		$ 1	$ 2
Stock Options, Weighted Average Grant Date Fair Value, Granted		$ 44.96	$ 27.42	$ 26.28
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)		3 years
Unrecognized compensation cost		$ 21
Weighted-average period, years		1 year
Estimated weighted average grant date fair value		$ 110.67	$ 75.98	$ 73.68
Intrinsic value of restricted stock units vested		$ 32	$ 37	$ 20
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)		3 years
Supplemental 401(k) Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Supplemental retirement savings plan, aggregate value of the accounts, shares		110,000	117,000
2025 Long-Term Incentive Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, additional shares authorized	1,400,000