Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 22.8	$ 19.6
Interest cost	34.2	39.6
Expected return on plan assets	(61.2)	(61.2)
Amortization:
Prior service cost	0.2	0.3
Net actuarial loss	36.8	19.5
Settlements	(0.7)	(0.2)
Net periodic benefit credit	32.1	17.6
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.3	0.2
Interest cost	0.4	0.6
Amortization:
Prior service cost	(1.4)	(1.4)
Net actuarial loss	0.4	0.2
Net periodic benefit credit	$ (0.3)	$ (0.4)