Accumulated Other Comprehensive Loss (Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 7,213.4	$ 7,381.1	$ 7,521.4
Other comprehensive income (loss) before reclassifications	340.9	(311.5)	(1,012.8)
Amounts reclassified from AOCL	192.2	53.9	117.8
Total Other Comprehensive Income (Loss)	533.1	(257.6)	(895.0)
Spin-off of Versum	11.5
Amount attributable to noncontrolling interest	3.7	4.8	(11.0)
Ending Balance	10,185.5	7,213.4	7,381.1
Derivatives qualifying as hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(65.0)	(42.9)	(28.5)
Other comprehensive income (loss) before reclassifications	(12.6)	13.7	(35.0)
Amounts reclassified from AOCL	24.2	(36.0)	20.8
Total Other Comprehensive Income (Loss)	11.6	(22.3)	(14.2)
Spin-off of Versum	0.2
Amount attributable to noncontrolling interest	(0.1)	(0.2)	0.2
Ending Balance	(53.1)	(65.0)	(42.9)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(949.3)	(956.5)	(268.7)
Other comprehensive income (loss) before reclassifications	101.9	9.9	(699.3)
Amounts reclassified from AOCL	57.3	2.7	0.0
Total Other Comprehensive Income (Loss)	159.2	12.6	(699.3)
Spin-off of Versum	6.0
Amount attributable to noncontrolling interest	3.0	5.4	(11.5)
Ending Balance	(787.1)	(949.3)	(956.5)
Pension and postretirement benefits
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,374.0)	(1,126.5)	(944.7)
Other comprehensive income (loss) before reclassifications	251.6	(335.1)	(278.5)
Amounts reclassified from AOCL	110.7	87.2	97.0
Total Other Comprehensive Income (Loss)	362.3	(247.9)	(181.5)
Spin-off of Versum	5.3
Amount attributable to noncontrolling interest	0.8	(0.4)	0.3
Ending Balance	(1,007.2)	(1,374.0)	(1,126.5)
AOCL attributable to Air Products
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,388.3)	(2,125.9)	(1,241.9)
Total Other Comprehensive Income (Loss)	529.4	(262.4)	(884.0)
Ending Balance	$ (1,847.4)	$ (2,388.3)	$ (2,125.9)