Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended		4 Months Ended	9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2023	Jun. 30, 2022	Sep. 30, 2022
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 9.4	$ 15.4		$ 30.6	$ 56.1
Interest cost	35.3	34.5		114.0	99.3
Expected return on plan assets	(44.0)	(56.4)		(147.0)	(175.1)
Amortization [Abstract]
Amortization of prior service credit	0.1	0.0		0.1	0.8
Amortization of net actuarial loss (gain)	0.2	10.4		(1.8)	55.0
Settlement and curtailment charges (benefit)	2.2	(0.7)		120.1	24.2
Net periodic postretirement benefit cost	$ 3.2	3.2		$ 116.0	60.3
Increase in net benefit obligation for remeasurement due to settlement			$ 25.3
Defined benefit plan, assumptions used calculating benefit obligation, discount rate	5.45%		5.45%	5.45%		5.65%
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	$ 0.2	0.2		$ 0.4	0.6
Interest cost	0.6	0.3		1.7	0.9
Amortization [Abstract]
Amortization of prior service credit	0.0	(0.2)		0.0	(0.6)
Amortization of net actuarial loss (gain)	0.1	0.2		0.4	0.5
Net periodic postretirement benefit cost	$ 0.9	$ 0.5		$ 2.5	$ 1.4