Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 594.2	$ 505.3	$ 449.9
Other comprehensive loss	(4.7)
Amounts reclassified from AOCI, net of tax	0.1	(0.1)	0.1
Other comprehensive loss	(4.6)	(1.7)	(8.6)
Ending balance	767.5	594.2	505.3
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	594.2
Ending balance		594.2
Foreign Currency Translation Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(31.5)
Other comprehensive loss	(4.5)
Amounts reclassified from AOCI, net of tax	0.0
Other comprehensive loss	(4.5)
Ending balance	(36.0)	(31.5)
Foreign Currency Translation Adjustment | Cumulative Effect, Period of Adoption, Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.0
Ending balance		0.0
Foreign Currency Translation Adjustment | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(31.5)
Ending balance		(31.5)
Pension Benefits Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3.3)
Other comprehensive loss	(0.2)
Amounts reclassified from AOCI, net of tax	0.1
Other comprehensive loss	(0.1)
Ending balance	(4.1)	(3.3)
Pension Benefits Adjustment | Cumulative Effect, Period of Adoption, Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.7)
Ending balance		(0.7)
Pension Benefits Adjustment | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(4.0)
Ending balance		(4.0)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(34.8)	(33.1)	(24.5)
Ending balance	(40.1)	(34.8)	$ (33.1)
Accumulated Other Comprehensive Loss | Cumulative Effect, Period of Adoption, Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.7)
Ending balance		(0.7)
Accumulated Other Comprehensive Loss | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (35.5)
Ending balance		$ (35.5)