Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020
Pension Benefits
Components of net periodic benefit cost (income)
Service cost			$ 23.1	$ 22.6	$ 68.7	$ 68.2
Interest cost			31.5	34.0	94.3	102.3
Expected return on plan assets			(60.9)	(60.9)	(182.0)	(183.3)
Amortization:
Prior service cost			0.3	0.2	1.0	0.7
Net actuarial loss			37.0	36.7	110.5	110.3
Settlements		$ (27.2)	27.2	(0.8)	26.8	(2.3)
Net periodic benefit credit			$ 58.2	31.8	$ 119.3	95.9
Decrease in net benefit obligation for remeasurement due to settlement	$ 500.9
Defined benefit plan, assumptions used calculating benefit obligation, discount rate	3.05%	3.05%	3.05%		3.05%		2.90%
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost			$ 0.3	0.3	$ 0.8	0.8
Interest cost			0.3	0.4	0.9	1.2
Amortization:
Prior service cost			(1.4)	(1.4)	(4.1)	(4.1)
Net actuarial loss			0.3	0.4	0.9	1.1
Net periodic benefit credit			$ (0.5)	$ (0.3)	$ (1.5)	$ (1.0)