Retirement Benefits - Schedule of Net Periodic Pension and Postretirement Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 10	$ 10	$ 28	$ 31
Interest cost	34	34	103	102
Expected return on plan assets	(42)	(42)	(127)	(125)
Amortization of net actuarial loss	3	7	11	20
Net periodic pension and postretirement benefit cost	5	9	15	28
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0	0	1	0
Interest cost	0	1	1	1
Amortization of net actuarial loss	1	0	2	2
Net periodic pension and postretirement benefit cost	$ 1	$ 1	$ 4	$ 3