Retirement Benefits - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 7.1	$ 6.8	$ 0.0
Pension plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.1	6.8
Pension plan | Other investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.4	4.1
Pension plan | Insurance contract | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2.7	$ 2.7