Goodwill and Other Intangible Assets - Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Other intangible assets
Amortized intangible assets, carrying amount	$ 470.5	$ 457.8
Amortized intangible assets, accumulated amortization	283.3	263.5
Amortized intangible assets, net	187.2	194.3
Total, carrying amount	514.2	501.5
Total, net	230.9	238.0
Goodwill and Other Intangible Assets (Textual) [Abstract]
Estimated amortization expense in 2018	27.9
Estimated amortization expense in 2019	24.9
Estimated amortization expense in 2020	21.9
Estimated amortization expense in 2021	21.1
Estimated amortization expense in 2022	19.1
Trademarks [Member]
Indefinite-lived Intangible Assets [Line Items]
Allen-Bradley® trademark not subject to amortization	43.7	43.7
Computer software products [Member]
Other intangible assets
Amortized intangible assets, carrying amount	197.7	194.8
Amortized intangible assets, accumulated amortization	119.1	113.2
Amortized intangible assets, net	78.6	81.6
Customer relationships [Member]
Other intangible assets
Amortized intangible assets, carrying amount	118.0	114.5
Amortized intangible assets, accumulated amortization	67.2	61.5
Amortized intangible assets, net	50.8	53.0
Technology [Member]
Other intangible assets
Amortized intangible assets, carrying amount	109.0	104.8
Amortized intangible assets, accumulated amortization	62.7	57.9
Amortized intangible assets, net	46.3	46.9
Trademarks [Member]
Other intangible assets
Amortized intangible assets, carrying amount	33.8	32.3
Amortized intangible assets, accumulated amortization	23.9	21.1
Amortized intangible assets, net	9.9	11.2
Other [Member]
Other intangible assets
Amortized intangible assets, carrying amount	12.0	11.4
Amortized intangible assets, accumulated amortization	10.4	9.8
Amortized intangible assets, net	$ 1.6	$ 1.6