Revenue Recognition - Schedule of Disaggregates Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2022		Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation Of Revenue [Line Items]
Net sales	$ 5,519.7		$ 4,816.3	$ 15,854.0	$ 13,655.6
Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,382.5		2,154.7	6,921.5	6,196.6
Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,270.2		1,132.2	3,659.5	3,275.3
Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,610.3		1,299.2	4,501.0	3,520.7
Distribution [Member]
Disaggregation Of Revenue [Line Items]
Net sales	357.7		322.3	1,044.8	906.4
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(101.0)		(92.1)	(272.8)	(243.4)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(93.5)		(86.9)	(246.2)	(229.9)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(6.1)		(5.0)	(19.0)	(13.0)
Intersegment Sales [Member] | Distribution [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(1.4)		(0.2)	(7.6)	(0.5)
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	4,517.3		3,939.5	12,947.6	11,181.3
U.S. [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,111.9		1,930.4	6,149.0	5,571.0
U.S. [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	740.5		630.3	2,108.8	1,834.4
U.S. [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,452.8		1,185.0	4,042.2	3,215.4
U.S. [Member] | Distribution [Member]
Disaggregation Of Revenue [Line Items]
Net sales	310.6		283.6	914.3	798.7
U.S. [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(98.5)		(89.8)	(266.7)	(238.2)
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	346.2		300.0	999.3	887.1
Canada [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	149.5		127.6	435.8	378.8
Canada [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	133.5		119.6	376.2	352.4
Canada [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	60.7		48.5	180.8	147.1
Canada [Member] | Distribution [Member]
Disaggregation Of Revenue [Line Items]
Net sales	4.8		6.4	12.1	13.6
Canada [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(2.3)		(2.1)	(5.6)	(4.8)
Latin America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	274.6		195.1	769.9	518.8
Latin America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	118.1		95.2	330.4	243.3
Latin America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	49.6		35.7	143.4	120.5
Latin America [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	64.7		32.1	178.0	61.2
Latin America [Member] | Distribution [Member]
Disaggregation Of Revenue [Line Items]
Net sales	42.3		32.3	118.4	94.1
Latin America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(0.1)		(0.2)	(0.3)	(0.3)
EMEA [Member]
Disaggregation Of Revenue [Line Items]
Net sales	291.3	[1]	287.0	857.1	804.1
EMEA [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3.0	[1]	1.5	6.3	3.5
EMEA [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	270.8	[1]	269.5	799.9	752.3
EMEA [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	17.6	[1]	16.0	51.1	48.3
EMEA [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(0.1)	[1]		(0.2)
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	90.3		94.7	280.1	264.3
Asia Pacific [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	75.8		77.1	231.2	215.7
Asia Pacific [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 14.5		$ 17.6	$ 48.9	48.7
Asia Pacific [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales					$ (0.1)

[1] Europe, Middle East and Africa ("EMEA")