Plant and Equipment, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 955.2	$ 884.7
Less: accumulated depreciation	624.9	588.2
Plant and equipment, net	330.3	296.5	$ 301.1
Land
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 21.8	21.9
Buildings
Property, Plant and Equipment [Line Items]
Useful Life in years	30 years
Plant and equipment, at cost	$ 149.3	147.1
Machinery and Equipment
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	732.0	689.5
Production facilities
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 479.9	433.6
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	$ 252.1	255.9
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	$ 52.1	$ 26.2