Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2021
Pension Benefits
Components of net periodic benefit cost (income)
Service cost		$ 20.3	$ 22.9	$ 40.7	$ 45.6
Interest cost		32.4	31.5	64.8	62.8
Expected return on plan assets		(59.3)	(60.7)	(118.7)	(121.1)
Amortization:
Prior service cost		0.8	0.3	0.8	0.7
Net actuarial loss		22.3	36.8	44.6	73.5
Settlements		24.9	(0.2)	24.9	(0.4)
Net periodic benefit cost (credit)		$ 41.4	30.6	$ 57.1	61.1
Defined benefit plan, benefit obligation, payment for settlement	$ 24.9
Decrease in net benefit obligation for remeasurement due to settlement	$ 199.2
Defined benefit plan, assumptions used calculating benefit obligation, discount rate	400.00%	400.00%		400.00%		310.00%
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost		$ 0.2	0.2	$ 0.4	0.5
Interest cost		0.3	0.3	0.6	0.6
Amortization:
Prior service cost		(0.2)	(1.3)	(0.4)	(2.7)
Net actuarial loss		0.1	0.3	0.3	0.6
Net periodic benefit cost (credit)		$ 0.4	$ (0.5)	$ 0.9	$ (1.0)