Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Components of net periodic benefit cost (income)
Service cost	$ 22.2	$ 24.1
Interest cost	38.8	37.8
Expected return on plan assets	(61.2)	(56.2)
Amortization:
Prior service cost (credit)	0.2	(0.8)
Net actuarial loss	28.3	38.0
Settlements	0.0	0.2
Net periodic benefit cost (income)	28.3	43.1
Other Postretirement Benefits [Member]
Components of net periodic benefit cost (income)
Service cost	0.3	0.3
Interest cost	0.6	0.6
Amortization:
Prior service cost (credit)	(1.3)	(1.5)
Net actuarial loss	0.4	0.5
Net periodic benefit cost (income)	$ 0.0	$ (0.1)