Retirement Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Amortization:
Prior service credit	$ (9.8)
Net actuarial loss	155.2
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost	97.0	$ 88.0	$ 85.7
Interest cost	151.6	169.5	167.2
Expected return on plan assets	(225.2)	(218.3)	(223.2)
Amortization:
Prior service credit	(3.7)	(2.9)	(2.7)
Net actuarial loss	152.9	124.5	118.7
Special termination benefit	0.5	0.5	0.0
Settlements	2.8	0.0	0.0
Net periodic benefit cost (income)	175.9	161.3	145.7
Other Postretirement Benefits [Member]
Components of net periodic benefit cost
Service cost	1.4	1.3	1.5
Interest cost	2.5	3.3	4.1
Amortization:
Prior service credit	(6.1)	(11.1)	(14.5)
Net actuarial loss	2.3	2.3	4.5
Special termination benefit	0.0	0.0	0.0
Settlements	0.0	0.0	0.0
Net periodic benefit cost (income)	$ 0.1	$ (4.2)	$ (4.4)