Business Segment Information - Sales and Operating Results of Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
Sales and operating results of reportable segments
Sales	$ 1,657.2	$ 1,651.2		$ 3,299.5	$ 3,237.8
Purchase accounting depreciation and amortization	(4.3)	(4.5)		(8.4)	(8.9)
General corporate – net	(26.7)	(24.8)		(48.6)	(48.8)
Non-operating pension and postretirement benefit credit (cost)	2.6	(5.7)		5.2	(11.5)
Costs related to unsolicited Emerson proposals	0.0	0.0	$ (11.2)	0.0	(11.2)
Gain (loss) on investments	98.2	0.0		(148.2)	0.0
Valuation adjustments related to the registration of PTC securities	0.0	0.0		33.7	0.0
Interest (expense) income - net	(21.2)	(10.3)		(39.2)	(22.5)
Income before income taxes	402.4	299.6		523.2	597.4
Operating Segments
Sales and operating results of reportable segments
Sales	1,657.2	1,651.2		3,299.5	3,237.8
Segment operating earnings	353.8	344.9		728.7	700.3
Architecture & Software
Sales and operating results of reportable segments
Sales	739.7			1,492.8
Architecture & Software \| Operating Segments
Sales and operating results of reportable segments
Sales	739.7	756.5		1,492.8	1,492.1
Segment operating earnings	209.9	217.2		446.9	441.4
Control Products & Solutions
Sales and operating results of reportable segments
Sales	917.5			1,806.7
Control Products & Solutions \| Operating Segments
Sales and operating results of reportable segments
Sales	917.5	894.7		1,806.7	1,745.7
Segment operating earnings	$ 143.9	$ 127.7		$ 281.8	$ 258.9

Equity Securities, FV-NI, Unrealized Gain (Loss), Loss On Investment

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Equity Securities, FV-NI, Unrealized Gain (Loss), Valuation Adjustment

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General Corporate - Net

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Non-operating pension costs consist of defined benefit plan interest cost, expected return on plan assets, amortization of actuarial gains and losses and the impacts of any plan curtailments or settlements. These costs are excluded from segment operating earnings.

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Purchase accounting depreciation and amortization that is not allocated to the segments for the purposes of determining segment operating earnings.

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Unsolicited Emerson Proposal

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Amount of interest revenue (income derived from investments in debt securities and on cash and cash equivalents) net of interest expense (cost of borrowed funds accounted for as interest).

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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