Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 9.2	$ 10.6	$ 18.5	$ 21.2
Interest cost	36.7	39.5	73.3	78.7
Expected return on plan assets	(42.5)	(51.7)	(84.7)	(103.0)
Amortization of net actuarial gain	(0.2)	(1.0)	(0.5)	(2.0)
Settlement charge	0.0	117.9	0.0	117.9
Net periodic postretirement benefit cost	3.2	115.3	6.6	112.8
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.2	0.1	0.3	0.2
Interest cost	0.7	0.5	1.3	1.1
Amortization of net actuarial gain	0.4	0.2	0.8	0.3
Net periodic postretirement benefit cost	$ 1.3	$ 0.8	$ 2.4	$ 1.6