Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Components of net periodic benefit cost (income)
Service cost	$ 22.4	$ 24.2	$ 44.6	$ 48.3
Interest cost	39.0	37.8	77.8	75.6
Expected return on plan assets	(61.5)	(56.2)	(122.7)	(112.4)
Amortization:
Prior service cost (credit)	0.1	(1.1)	0.3	(1.9)
Net actuarial loss	28.4	38.2	56.7	76.2
Settlements	0.0	0.0	0.0	0.2
Net periodic benefit cost	28.4	42.9	56.7	86.0
Other Postretirement Benefits [Member]
Components of net periodic benefit cost (income)
Service cost	0.3	0.4	0.6	0.7
Interest cost	0.6	0.7	1.2	1.3
Amortization:
Prior service cost (credit)	(1.4)	(1.5)	(2.7)	(3.0)
Net actuarial loss	0.5	0.6	0.9	1.1
Net periodic benefit cost	$ 0.0	$ 0.2	$ 0.0	$ 0.1