Business Segment Information - Sales and Operating Results of Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Sales and operating results of reportable segments
Sales	$ 1,698.7		$ 1,599.2	$ 4,936.5	$ 4,643.8
Purchase accounting depreciation and amortization	(4.3)		(5.6)	(13.2)	(16.8)
General corporate – net	(27.7)		(16.5)	(61.7)	(52.8)
Non-operating pension costs	(5.9)		(19.8)	(17.7)	(59.4)
Costs related to unsolicited Emerson proposals	0.0		0.0	(11.2)	0.0
Loss on investments	(7.3)		0.0	(7.3)	0.0
Valuation adjustment pending registration of PTC securities	(69.5)		0.0	(69.5)	0.0
Interest expense	(16.5)		(19.1)	(53.8)	(56.7)
Income before income taxes	251.7		276.0	849.1	763.9
Third-party advisory fees		$ 11.2
Operating Segments [Member]
Sales and operating results of reportable segments
Sales	1,698.7		1,599.2	4,936.5	4,643.8
Segment operating earnings	382.9		337.0	1,083.5	949.6
Architecture & Software [Member] | Operating Segments [Member]
Sales and operating results of reportable segments
Sales	795.7		731.9	2,311.0	2,147.3
Segment operating earnings	239.0		204.3	681.8	603.5
Control Products & Solutions [Member] | Operating Segments [Member]
Sales and operating results of reportable segments
Sales	903.0		867.3	2,625.5	2,496.5
Segment operating earnings	$ 143.9		$ 132.7	$ 401.7	$ 346.1