Employee Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss), Pre-Tax (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, pension and other postretirement tax benefit (provision)		$ 2
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, pension and other postretirement tax benefit (provision)	$ 1		$ (1)