Goodwill and Other Intangible Assets - Other Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Total, carrying amount	$ 504.1	$ 501.5
Total, net	234.0	238.0
Other intangible assets
Amortized intangible assets, carrying amount	460.4	457.8
Amortized intangible assets, accumulated amortization	270.1	263.5
Amortized intangible assets, net	190.3	194.3
Goodwill and Other Intangible Assets (Textual) [Abstract]
Estimated amortization expense in 2018	27.7
Estimated amortization expense in 2019	24.7
Estimated amortization expense in 2020	21.8
Estimated amortization expense in 2021	21.0
Estimated amortization expense in 2022	19.0
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	194.8	194.8
Amortized intangible assets, accumulated amortization	116.1	113.2
Amortized intangible assets, net	78.7	81.6
Customer relationships [Member]
Other intangible assets
Amortized intangible assets, carrying amount	114.4	114.5
Amortized intangible assets, accumulated amortization	62.8	61.5
Amortized intangible assets, net	51.6	53.0
Technology [Member]
Other intangible assets
Amortized intangible assets, carrying amount	107.4	104.8
Amortized intangible assets, accumulated amortization	59.5	57.9
Amortized intangible assets, net	47.9	46.9
Trademarks [Member]
Other intangible assets
Amortized intangible assets, carrying amount	32.4	32.3
Amortized intangible assets, accumulated amortization	21.8	21.1
Amortized intangible assets, net	10.6	11.2
Other [Member]
Other intangible assets
Amortized intangible assets, carrying amount	11.4	11.4
Amortized intangible assets, accumulated amortization	9.9	9.8
Amortized intangible assets, net	$ 1.5	$ 1.6