Employee Retirement Plans - Net periodic pension (income) cost (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 9	$ 16	$ 12
Interest cost	35	26	22
Expected return on assets	(45)	(50)	(40)
Amortization of:
Actuarial (gain) loss	(3)	7	14
Net benefit (income) cost	(4)	(1)	$ 8
Contribution made to Pension Plan	$ 19	$ 15