Plant and Equipment, Net (Major Classes of Plant and Equipment) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 42,754.8	$ 39,950.9
Less: Accumulated depreciation	17,417.0	16,580.0
Plant and equipment, net	25,337.8	23,370.9	$ 17,472.1
Land
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	343.0	312.1
Buildings
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 1,766.1	1,730.7
Useful life (in years)	30 years
Production facilities
Property, Plant and Equipment [Line Items]
Plant and equipment, at cost	$ 21,412.9	21,245.2
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	$ 5,776.0	5,472.7
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	$ 13,456.8	$ 11,190.2