Acquisitions (Narrative) (Details) $ in Millions, ¥ in Billions			3 Months Ended	9 Months Ended
	Apr. 26, 2018 USD ($)	Apr. 26, 2018 CNY (¥)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 CNY (¥)	Sep. 30, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Plant and equipment, net			$ 9,902.0	$ 9,902.0			$ 8,440.2
Additions to plant and equipment				1,158.1	$ 806.8
Equity issued related to asset acquisition			227.4	227.4
Long-term debt – related party			398.7	398.7		¥ 2.6	$ 0.0
Liability for expected cash payments to or on behalf of related party			$ 345.0	345.0		¥ 2.3
Business Acquisition [Line Items]
Business combinations, goodwill acquired during period				$ 81.4
Loan | Lu'An Loan To Joint Venture
Property, Plant and Equipment [Line Items]
Fixed interest rate (percent)			5.50%	5.50%		5.50%
Joint Venture | Air Products Lu'an Changzhi Co Ltd
Property, Plant and Equipment [Line Items]
Consideration transferred in asset acquisition	$ 1,200.0	¥ 7.9
Joint Venture | Air Products Lu'an Changzhi Co Ltd
Property, Plant and Equipment [Line Items]
Ownership interest by Air Products (percent)	60.00%
Ownership interest by noncontrolling owners (percent)	40.00%
Contribution of equipment to joint venture	$ 300.0
Plant and equipment, net	1,500.0
Additions to plant and equipment	235.0	1.5
Equity issued related to asset acquisition	$ 227.0	¥ 1.4
2018 Business Combinations
Business Acquisition [Line Items]
Number of business combinations completed				8
Business combinations, aggregate purchase price, net of cash acquired				$ 355.4
Business combinations, plant and equipment acquired during the period			$ 178.4	178.4
Business combinations, goodwill acquired during period				81.4
Business combinations, tax deductible goodwill			18.0	18.0
Business combinations, recognized identifiable intangible assets			$ 105.8	$ 105.8
Acquired finite-lived intangible assets, weighted-average useful life				12 years