Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2022	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Sep. 30, 2021
Pension Benefits
Components of net periodic benefit cost (income)
Service cost		$ 15.4	$ 23.1	$ 56.1	$ 68.7
Interest cost		34.5	31.5	99.3	94.3
Expected return on plan assets		(56.4)	(60.9)	(175.1)	(182.0)
Amortization:
Prior service cost		0.0	0.3	0.8	1.0
Net actuarial loss		10.4	37.0	55.0	110.5
Settlement and curtailment (benefit) charges		(0.7)	27.2	24.2	26.8
Net periodic benefit cost (credit)		3.2	58.2	60.3	119.3
Defined benefit plan, benefit obligation, payment for settlement		$ 6.5		$ 31.4
Decrease in net benefit obligation for remeasurement due to settlement	$ 86.6
Defined benefit plan, assumptions used calculating benefit obligation, discount rate		490.00%		490.00%		310.00%
Curtailment benefit		$ 7.2		$ 7.2
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost		0.2	0.3	0.6	0.8
Interest cost		0.3	0.3	0.9	0.9
Amortization:
Prior service cost		(0.2)	(1.4)	(0.6)	(4.1)
Net actuarial loss		0.2	0.3	0.5	0.9
Net periodic benefit cost (credit)		$ 0.5	$ (0.5)	$ 1.4	$ (1.5)