Employee Retirement Plans - Net periodic pension (income) cost (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8	$ 9	$ 16
Interest cost	36	35	26
Expected return on assets	(44)	(45)	(50)
Amortization of:
Actuarial loss (gain)	1	(3)	7
Net benefit cost (income)	$ 1	$ (4)	$ (1)