Plant and Equipment, Net (Major Classes of Plant and Equipment) (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		$ 19,547.8	$ 18,660.2
Less: accumulated depreciation		11,107.6	10,400.5
Plant and equipment, net	[1]	8,440.2	8,259.7	$ 8,237.3
Land
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost		231.0	202.9
Buildings
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost		$ 977.8	918.6
Useful life (in years)		30 years
Production facilities
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	[2]	$ 13,577.1	12,391.9
Production facilities | Minimum
Property, Plant and Equipment [Line Items]
Useful life (in years)	[2]	10 years
Production facilities | Maximum
Property, Plant and Equipment [Line Items]
Useful life (in years)	[2]	20 years
Distribution and other machinery and equipment
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	[3]	$ 3,944.0	3,821.0
Distribution and other machinery and equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful life (in years)	[3]	5 years
Distribution and other machinery and equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful life (in years)	[3]	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
Tanks
Property, Plant and Equipment [Line Items]
Useful life (in years)		20 years
Customer Stations
Property, Plant and Equipment [Line Items]
Useful life (in years)		7 years 6 months
Tractors and Trailers | Minimum
Property, Plant and Equipment [Line Items]
Useful life (in years)		5 years
Tractors and Trailers | Maximum
Property, Plant and Equipment [Line Items]
Useful life (in years)		15 years
Construction in progress
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost		$ 817.9	$ 1,325.8

[1] Long-lived assets include plant and equipment, net.

[2] Depreciable lives of production facilities related to long-term customer supply contracts are matched to the contract lives.

[3] The depreciable lives for various types of distribution equipment are 10 to 25 years for cylinders, depending on the nature and properties of the product; 20 years for tanks; 7.5 years for customer stations; and 5 to 15 years for tractors and trailers.