Goodwill and Other Intangible Assets - Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018
Other intangible assets
Amortized intangible assets, carrying amount	$ 459.3	$ 453.8
Amortized intangible assets, accumulated amortization	295.3	282.3
Amortized intangible assets, net	164.0	171.5
Total, carrying amount	503.0	497.5
Total, net	207.7	215.2
Goodwill and Other Intangible Assets (Textual) [Abstract]
Estimated amortization expense in 2019	25.3
Estimated amortization expense in 2020	22.7
Estimated amortization expense in 2021	22.0
Estimated amortization expense in 2022	20.0
Estimated amortization expense in 2023	18.8
Trademarks
Indefinite-lived Intangible Assets [Line Items]
Allen-Bradley® trademark not subject to amortization	43.7	43.7
Computer software products
Other intangible assets
Amortized intangible assets, carrying amount	190.6	190.9
Amortized intangible assets, accumulated amortization	123.3	118.1
Amortized intangible assets, net	67.3	72.8
Customer relationships
Other intangible assets
Amortized intangible assets, carrying amount	113.0	112.9
Amortized intangible assets, accumulated amortization	69.0	66.2
Amortized intangible assets, net	44.0	46.7
Technology
Other intangible assets
Amortized intangible assets, carrying amount	112.1	106.8
Amortized intangible assets, accumulated amortization	67.3	64.0
Amortized intangible assets, net	44.8	42.8
Trademarks
Other intangible assets
Amortized intangible assets, carrying amount	32.6	32.0
Amortized intangible assets, accumulated amortization	25.8	24.0
Amortized intangible assets, net	6.8	8.0
Other
Other intangible assets
Amortized intangible assets, carrying amount	11.0	11.2
Amortized intangible assets, accumulated amortization	9.9	10.0
Amortized intangible assets, net	$ 1.1	$ 1.2