Plant and Equipment, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 1,072.1	$ 955.2
Less: accumulated depreciation	667.0	624.9
Plant and equipment, net	405.1	330.3	$ 296.5
Land
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 22.1	21.8
Buildings
Property, Plant and Equipment [Line Items]
Useful Life in years	30 years
Plant and equipment, at cost	$ 168.6	149.3
Machinery and Equipment
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	815.2	732.0
Production facilities
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 467.4	479.9
Production facilities | Minimum
Property, Plant and Equipment [Line Items]
Useful Life in years	10 years
Production facilities | Maximum
Property, Plant and Equipment [Line Items]
Useful Life in years	15 years
Distribution and other
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 347.8	252.1
Distribution and other | Minimum
Property, Plant and Equipment [Line Items]
Useful Life in years	5 years
Distribution and other | Maximum
Property, Plant and Equipment [Line Items]
Useful Life in years	25 years
Cylinders | Minimum
Property, Plant and Equipment [Line Items]
Useful Life in years	10 years
Cylinders | Maximum
Property, Plant and Equipment [Line Items]
Useful Life in years	25 years
Other distribution equipment
Property, Plant and Equipment [Line Items]
Useful Life in years	20 years
Construction in progress
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 66.2	$ 52.1