Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 22.8	$ 19.5	$ 45.6	$ 39.1
Interest cost	34.1	39.6	68.3	79.2
Expected return on plan assets	(61.2)	(61.2)	(122.4)	(122.4)
Amortization:
Prior service cost	0.3	0.3	0.5	0.6
Net actuarial loss	36.8	19.5	73.6	39.0
Settlements	(0.7)	(0.2)	(1.5)	(0.4)
Net periodic benefit credit	32.1	17.5	64.1	35.1
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.2	0.3	0.5	0.5
Interest cost	0.4	0.5	0.8	1.1
Amortization:
Prior service cost	(1.3)	(1.3)	(2.7)	(2.7)
Net actuarial loss	0.3	0.2	0.7	0.4
Net periodic benefit credit	$ (0.4)	$ (0.3)	$ (0.7)	$ (0.7)