Accumulated Other Comprehensive Loss - Reclassifications out of Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Sales	$ 1,565.3	$ 1,684.5
Cost of sales	(918.8)	(981.6)
Selling, general and administrative expenses	(374.6)	(403.2)
Interest expense	(22.6)	(26.4)
Income before income taxes	700.7	334.6
Income tax provision	(110.3)	(19.2)
Net income attributable to Rockwell Automation	593.3	310.7
Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Rockwell Automation	30.7	24.0
Pension and other postretirement benefit plan adjustments | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	35.8	35.3
Income tax provision	(8.2)	(7.9)
Net income attributable to Rockwell Automation	27.6	27.4
Amortization of prior service credit | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(1.0)	(1.2)
Amortization of net actuarial loss | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	37.0	37.2
Settlements | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(0.2)	(0.7)
Net unrealized gains (losses) on cash flow hedges, net of tax | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	4.3	(4.5)
Income tax provision	(1.2)	1.1
Net income attributable to Rockwell Automation	3.1	(3.4)
Net unrealized gains (losses) on cash flow hedges, net of tax | Forward exchange contracts | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Sales	(0.5)	(0.1)
Cost of sales	4.7	(5.0)
Selling, general and administrative expenses	(0.4)	0.1
Net unrealized gains (losses) on cash flow hedges, net of tax | Treasury locks related to 2019 debt issuance | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ 0.5	$ 0.5