Retirement Benefits - Components of Net Periodic Pension Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 2.0	$ 2.3
Interest cost	0.7	0.6
Expected return on plan assets	(0.1)	(0.1)
Actuarial loss amortization	0.1	0.5
Net periodic pension cost	$ 2.7	$ 3.3