Retirement Benefits (Summary of Changes in Pension Plan Assets Fair Value Classified as Level 3) (Details) - Defined Benefit Pension Plan - Level 3 - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value at beginning of year	$ 245.2	$ 226.1
Actual return on plan assets:
Assets held at end of year	15.9	21.2
Assets sold during the period	0.3	0.3
Purchases, sales, and settlements, net	(4.4)	(2.4)
Fair value at end of year	257.0	245.2
Real Estate Pooled Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value at beginning of year	192.1	174.2
Actual return on plan assets:
Assets held at end of year	15.7	17.9
Assets sold during the period	0.0	0.0
Purchases, sales, and settlements, net	0.0	0.0
Fair value at end of year	207.8	192.1
Other Pooled Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value at beginning of year	7.3	6.6
Actual return on plan assets:
Assets held at end of year	1.2	0.1
Assets sold during the period	0.3	0.3
Purchases, sales, and settlements, net	(1.0)	0.3
Fair value at end of year	7.8	7.3
Insurance Contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value at beginning of year	45.8	45.3
Actual return on plan assets:
Assets held at end of year	(1.0)	3.2
Assets sold during the period	0.0	0.0
Purchases, sales, and settlements, net	(3.4)	(2.7)
Fair value at end of year	$ 41.4	$ 45.8