Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 19.5	$ 22.4	$ 39.1	$ 44.6
Interest cost	39.6	39.0	79.2	77.8
Expected return on plan assets	(61.2)	(61.5)	(122.4)	(122.7)
Amortization:
Prior service cost	0.3	0.1	0.6	0.3
Net actuarial loss	19.5	28.4	39.0	56.7
Settlements	(0.2)	0.0	(0.4)	0.0
Net periodic benefit credit	17.5	28.4	35.1	56.7
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.3	0.3	0.5	0.6
Interest cost	0.5	0.6	1.1	1.2
Amortization:
Prior service cost	(1.3)	(1.4)	(2.7)	(2.7)
Net actuarial loss	0.2	0.5	0.4	0.9
Net periodic benefit credit	$ (0.3)	$ 0.0	$ (0.7)	$ 0.0