Business Segment Information - Schedule of Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Sales	$ 1,570.0	$ 1,394.0	$ 1,681.3	$ 1,684.5	$ 1,730.2	$ 1,665.1	$ 1,657.2	$ 1,642.3	$ 6,329.8	$ 6,694.8	$ 6,666.0
Sales and operating results of reportable segments
Purchase accounting depreciation and amortization									(41.4)	(16.6)	(17.4)
General corporate - net									(98.9)	(108.8)	(100.0)
Non-operating pension and postretirement benefit (cost) credit									(37.4)	8.4	(23.8)
Costs related to unsolicited Emerson proposals									0.0	0.0	(11.2)
Gain (loss) on investments									153.9	(402.2)	123.7
Valuation adjustments related to the registration of PTC Shares									0.0	33.7	(33.7)
Interest (expense) income - net									98.0	87.1	48.6
Income before income taxes									1,136.1	901.0	1,330.8
Operating Segments
Segment Reporting Information [Line Items]
Sales									6,329.8	6,694.8	6,666.0
Operating Income (Loss)									1,257.9	1,473.6	1,441.8
Architecture & Software
Segment Reporting Information [Line Items]
Sales									2,832.9	3,021.9
Architecture & Software | Operating Segments
Segment Reporting Information [Line Items]
Sales									2,832.9	3,021.9	3,050.2
Operating Income (Loss)									795.2	874.8	897.9
Control Products & Solutions
Segment Reporting Information [Line Items]
Sales									3,496.9	3,672.9
Control Products & Solutions | Operating Segments
Segment Reporting Information [Line Items]
Sales									3,496.9	3,672.9	3,615.8
Operating Income (Loss)									$ 462.7	$ 598.8	$ 543.9