Business Segment Information - Schedule of Reportable Segments (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Sep. 30, 2017	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
Sales and operating results of reportable segments
Sales		$ 1,729.5	$ 1,698.7	$ 1,651.2	$ 1,586.6	$ 1,667.5	$ 1,599.2	$ 1,554.3	$ 1,490.3	$ 6,666.0	$ 6,311.3	$ 5,879.5
Income before income taxes		$ 481.7	$ 251.7	$ 299.6	$ 297.8	$ 273.5	$ 276.0	$ 230.3	$ 257.6	1,330.8	1,037.4	943.1
Purchase accounting depreciation and amortization										(17.4)	(21.4)	(18.4)
Gain on sale of business	$ 60.8									0.0	60.8	0.0
Costs related to unsolicited Emerson proposals										(11.2)	0.0	0.0
Gain on investments										123.7	0.0	0.0
Valuation adjustment pending registration of PTC Shares										(33.7)	0.0	0.0
Interest expense										(73.0)	(76.2)	(71.3)
Operating Segments [Member]
Sales and operating results of reportable segments
Income before income taxes										1,442.6	1,233.1	1,188.7
Segment Reconciling Items
Sales and operating results of reportable segments
Purchase accounting depreciation and amortization										(17.4)	(21.4)	(18.4)
Non-operating pension costs										(24.6)	(82.6)	(76.2)
Gain on sale of business										0.0	60.8	0.0
Corporate
Sales and operating results of reportable segments
General corporate-net										(75.6)	(76.3)	(79.7)
Architecture & Software
Sales and operating results of reportable segments
Sales										3,098.2	2,899.3	2,635.2
Income before income taxes										901.3	781.5	695.0
Control Products & Solutions
Sales and operating results of reportable segments
Sales										3,567.8	3,412.0	3,244.3
Income before income taxes										$ 541.3	$ 451.6	$ 493.7