RESTRUCTURING AND RELATED CHARGES - Summary of Restructuring and Related Charges (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges	$ 40.3	$ 71.6	$ 61.0
Cost of goods sold
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges	1.9	13.8	2.5
Operating expense
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges	38.4	57.8	58.5
Global productivity improvement program
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges	21.2	71.1	59.9
GPC Edwardsville 3PL transition
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges	11.5	0.0	0.0
SAP S4 ERP transformation
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges	4.3	0.0	0.0
Other restructuring activities
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges	$ 3.3	$ 0.5	$ 1.1