Business Segment and Geographic Information (Reconciliation of Operating Income) (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2016
Segment Reporting Information [Line Items]
Change in inventory valuation method													$ 2,015.0	$ 1,416.1		$ 1,554.6
Business separation costs					$ 0.0	[1]	$ 0.0	[1]	$ 0.0	[1]	$ (32.5)	[1]	0.0	(32.5)	[1]	(50.6)
Business restructuring and cost reduction actions					(48.4)	[2]	(42.7)	[2]	(10.3)	[2]	(50.0)	[2]	0.0	(151.4)	[2]	(34.5)
Goodwill and intangible asset impairment charge					0.0	[3]	(162.1)	[3]	0.0	[3]	0.0	[3]	0.0	(162.1)	[3]	0.0
Gain on land sale					12.2		0.0		0.0		0.0			12.2
Operating income	$ 533.7	$ 515.8	$ 455.4	$ 460.7	$ 457.4		$ 258.7		$ 395.6		$ 328.3		1,965.6	1,440.0		1,535.1
Segment Reconciling Items
Segment Reporting Information [Line Items]
Business separation costs													0.0	(32.5)		(50.6)
Business restructuring and cost reduction actions													0.0	(151.4)		(34.5)
Goodwill and intangible asset impairment charge													0.0	(162.1)		0.0
Gain on land sale													0.0	12.2		0.0
Segment Total
Segment Reporting Information [Line Items]
Operating income													1,941.5	1,773.8		1,620.2
Change in Inventory Valuation Method
Segment Reporting Information [Line Items]
Change in inventory valuation method	$ 24.1												24.1
Change in Inventory Valuation Method | Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in inventory valuation method													$ 24.1	$ 0.0		$ 0.0

[1] For additional information, refer to Note 4, Materials Technologies Separation.

[2] For additional information, refer to Note 5, Cost Reduction and Asset Actions.

[3] For additional information, refer to Note 10, Goodwill, and Note 11, Intangible Assets.