Plant and Equipment, Net (Major Classes of Plant and Equipment) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 27,488.8	$ 25,176.2
Less: Accumulated depreciation	14,234.2	13,211.5
Plant and equipment, net	13,254.6	11,964.7	$ 10,337.6
Land
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	312.1	296.8
Buildings
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 1,083.1	997.8
Useful life (in years)	30 years
Production facilities
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 18,236.9	17,289.7
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)	20 years
Distribution and other machinery and equipment
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 5,111.6	4,807.7
Distribution and other machinery and equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful life (in years)	5 years
Distribution and other machinery and equipment | 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
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	$ 2,745.1	$ 1,784.2