Retirement Benefits - Benefit Obligation, Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Change in benefit obligation:
beginning of year	$ 22.8	$ 0.0
Converted on October 1, 2016	0.0	26.5
Service cost	2.0	2.3
Interest cost	0.7	0.6
Actuarial loss (gain)	1.3	(4.9)
Benefits paid	(0.8)	(1.2)
Other	0.7	0.0
Foreign currency impact	0.4	(0.5)
Benefit obligation at end of year	27.1	22.8
Change in plan assets:
beginning of year	7.1	6.8	$ 7.1	$ 6.8	$ 0.0
Converted on October 1, 2016	0.0	3.0
Employer contributions	0.8	5.0
Benefits paid	(0.8)	(1.2)
Foreign currency impact	0.1	0.0
end of year	$ 7.1	$ 6.8
Funded status:
Plan assets less than benefit obligation - Net amount recognized			$ (20.0)	$ (16.0)