Revenue Recognition - Schedule of Disaggregates Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023		Dec. 31, 2022
Disaggregation Of Revenue [Line Items]
Net sales	$ 4,620.0		$ 4,923.1
Operating Segments [Member]
Disaggregation Of Revenue [Line Items]
Net sales	4,687.2		4,997.5
Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,419.9		2,337.4
Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,059.3		1,215.0
Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	918.3		1,123.6
Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	289.7		321.5
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(67.2)		(74.4)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(59.2)		(68.4)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(5.3)		(4.7)
Intersegment Sales [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(2.7)		(1.3)
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,397.8		3,904.4
U.S. [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,796.4		1,973.5
U.S. [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	593.8		703.0
U.S. [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	820.5		1,022.7
U.S. [Member] | Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	248.1		276.7
U.S. [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(61.0)		(71.5)
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	303.4		308.2
Canada [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	135.1		137.1
Canada [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	116.7		123.7
Canada [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	51.1		45.5
Canada [Member] | Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1.2		3.4
Canada [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(0.7)		(1.5)
Latin America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	551.3		350.9
Latin America [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	485.4		225.5
Latin America [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	6.1		51.7
Latin America [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	23.5		33.6
Latin America [Member] | Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	40.4		41.4
Latin America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(4.1)		(1.3)
EMEA [Member]
Disaggregation Of Revenue [Line Items]
Net sales	284.4	[1]	272.2
EMEA [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3.0	[1]	1.3
EMEA [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	270.9	[1]	259.9
EMEA [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	11.9	[1]	11.1
EMEA [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(1.4)	[1]	(0.1)
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	83.1		87.4
Asia Pacific [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	71.8		76.7
Asia Pacific [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 11.3		$ 10.7

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