Business Segment and Geographic Information (Reconciliation of Operating Income) (Details) - USD ($) $ in Millions		3 Months Ended																	12 Months Ended
		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2015
Segment Reporting Information [Line Items]
Business separation costs		$ 0.0	[1]	$ 0.0	[1]	$ 0.0	[1]	$ (30.2)	[1]	$ (21.7)	[1]	$ (9.5)	[1]	$ (7.4)	[1]	$ (12.0)	[1]		$ (30.2)	[1]	$ (50.6)	[1]	$ (7.5)
Business restructuring and cost reduction actions		(48.4)	[2]	(42.7)	[2]	(10.3)	[2]	(50.0)	[2]	(10.6)	[2]	(13.2)	[2]	(10.7)	[2]	0.0	[2]		(151.4)	[2]	(34.5)	[2]	(180.1)
Pension settlement loss		(0.9)	[3]	(5.5)	[3]	(4.1)	[3]	0.0	[3]	(2.1)	[3]	(1.0)	[3]	(2.0)	[3]	0.0	[3]		(10.5)	[3]	(5.1)	[3]	(19.3)
Goodwill and intangible asset impairment charge		0.0	[4]	(162.1)	[4]	0.0	[4]	0.0	[4]										(162.1)	[4]	0.0		0.0
Gain on previously held equity interest																			0.0		0.0		17.9
Gain on land sales		12.2	[5]	0.0	[5]	0.0	[5]	0.0	[5]									$ 33.6	12.2	[5]
Operating income		$ 455.7	[6]	$ 252.6	[6]	$ 391.2	[6]	$ 328.1	[6]	$ 391.0	[6]	$ 394.6	[6]	$ 371.6	[6]	$ 372.5	[6]		1,427.6	[6]	1,529.7	[6]	1,233.2
Segment Total
Segment Reporting Information [Line Items]
Operating income																			1,769.6		1,619.9		1,388.6
Segment Reconciling Items
Segment Reporting Information [Line Items]
Business separation costs																			(30.2)		(50.6)		(7.5)
Business restructuring and cost reduction actions																			(151.4)		(34.5)		(180.1)
Pension settlement loss																			(10.5)		(5.1)		(19.3)
Goodwill and intangible asset impairment charge																			(162.1)		0.0		0.0
Gain on previously held equity interest																			0.0		0.0		17.9
Gain on land sales	[7]																		$ 12.2		$ 0.0		$ 33.6

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

[2] For additional information, see Note 5, Business Restructuring and Cost Reduction Actions.

[3] For additional information, see Note 16, Retirement Benefits.

[4] For additional information, see Note 10, Goodwill, and Note 11, Intangible Assets.

[5] The gain is reflected on the consolidated income statements in "Other income (expense), net." For additional information, see Note 23, Supplemental Information.

[6] Changes in estimates on projects accounted for under the percentage of completion method favorably impacted income by approximately $27 in fiscal year 2017 and $20 in fiscal year 2016, primarily during the fourth quarter. For additional information, see Note 1, Major Accounting Policies (Revenue Recognition).

[7] Reflected on the consolidated income statements in “Other income (expense), net.”