Retirement Benefits - Benefit Obligation, Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
Change in benefit obligation:
September 30, 2016	$ 0.0
Converted on October 1, 2016	26.5
Service cost	2.3
Interest cost	0.6
Actuarial gain	(4.9)
Benefits paid	(1.2)
Foreign currency impact	(0.5)
Benefit obligation at end of year	22.8
Change in plan assets:
September 30, 2016	6.8	$ 6.8	$ 0.0
Converted on October 1, 2016	3.0
Employer contributions	5.0
Benefits paid	(1.2)
September 30, 2017	$ 6.8
Funded status:
Plan assets less than benefit obligation - Net amount recognized		$ (16.0)