Defined Benefit Pension and Other Postretirement Plans - Components of Net Periodic Pension Benefit Cost (Income) (Details) - Pension Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Defined Benefit Plan Disclosure
Service cost	$ 2	$ 3	$ 4	$ 5
Interest cost	9	8	18	17
Expected return on plan assets	(11)	(12)	(22)	(23)
Amortization of:
Actuarial loss (gain)	1	0	1	(1)
Net benefit cost (income)	$ 1	$ (1)	$ 1	$ (2)