Goodwill and Other Intangible Assets - Other Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Other intangible assets
Amortized intangible assets, carrying amount	$ 455.4	$ 457.8
Amortized intangible assets, accumulated amortization	276.6	263.5
Amortized intangible assets, net	178.8	194.3
Total, carrying amount	499.1	501.5
Total, net	222.5	238.0
Goodwill and Other Intangible Assets (Textual) [Abstract]
Estimated amortization expense in 2018	27.6
Estimated amortization expense in 2019	24.6
Estimated amortization expense in 2020	21.8
Estimated amortization expense in 2021	20.9
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	190.9	194.8
Amortized intangible assets, accumulated amortization	115.2	113.2
Amortized intangible assets, net	75.7	81.6
Customer relationships [Member]
Other intangible assets
Amortized intangible assets, carrying amount	113.8	114.5
Amortized intangible assets, accumulated amortization	65.3	61.5
Amortized intangible assets, net	48.5	53.0
Technology [Member]
Other intangible assets
Amortized intangible assets, carrying amount	107.2	104.8
Amortized intangible assets, accumulated amortization	62.8	57.9
Amortized intangible assets, net	44.4	46.9
Trademarks [Member]
Other intangible assets
Amortized intangible assets, carrying amount	32.2	32.3
Amortized intangible assets, accumulated amortization	23.4	21.1
Amortized intangible assets, net	8.8	11.2
Other [Member]
Other intangible assets
Amortized intangible assets, carrying amount	11.3	11.4
Amortized intangible assets, accumulated amortization	9.9	9.8
Amortized intangible assets, net	$ 1.4	$ 1.6