Business Segment Information - Sales and Operating Results of Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Sales and operating results of reportable segments
Sales	$ 1,651.2		$ 1,554.3	$ 3,237.8	$ 3,044.6
Purchase accounting depreciation and amortization	(4.5)		(5.6)	(8.9)	(11.2)
General corporate – net	(17.8)		(21.4)	(34.0)	(36.3)
Non-operating pension costs	(5.9)		(19.8)	(11.8)	(39.6)
Costs related to unsolicited Emerson proposals	0.0		0.0	(11.2)	0.0
Interest expense	(17.3)		(18.9)	(37.3)	(37.6)
Income before income taxes	299.6		230.3	597.4	487.9
Third-party advisory fees		$ 11.2
Operating Segments [Member]
Sales and operating results of reportable segments
Sales	1,651.2		1,554.3	3,237.8	3,044.6
Operating Income (Loss)	345.1		296.0	700.6	612.6
Architecture & Software [Member] | Operating Segments [Member]
Sales and operating results of reportable segments
Sales	768.4		719.0	1,515.3	1,415.4
Operating Income (Loss)	218.2		190.6	442.8	399.2
Control Products & Solutions [Member] | Operating Segments [Member]
Sales and operating results of reportable segments
Sales	882.8		835.3	1,722.5	1,629.2
Operating Income (Loss)	$ 126.9		$ 105.4	$ 257.8	$ 213.4