Revenue Recognition -Schedule of Disaggregates Revenue by Primary Geographical Markets (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2023		Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Disaggregation Of Revenue [Line Items]
Net sales	$ 5,277.6		$ 5,382.1	$ 10,200.7	$ 10,334.3
Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,627.4		2,319.0	4,964.8	4,539.0
Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,265.1		1,250.6	2,480.1	2,389.3
Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,168.2		1,538.1	2,291.8	2,890.7
Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	307.3		362.3	628.8	687.1
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(90.4)		(87.9)	(164.8)	(171.8)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(81.6)		(78.5)	(150.0)	(152.7)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(7.5)		(6.9)	(12.2)	(12.9)
Intersegment Sales [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(1.3)		(2.5)	(2.6)	(6.2)
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,975.3		4,397.7	7,879.7	8,430.3
U.S. [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,001.6		2,068.1	3,975.1	4,037.1
U.S. [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	741.7		720.0	1,444.7	1,368.3
U.S. [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,056.3		1,376.3	2,079.0	2,589.4
U.S. [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	260.3		319.1	537.0	603.7
U.S. [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(84.6)		(85.8)	(156.1)	(168.2)
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	327.9		338.3	636.1	653.1
Canada [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	138.0		145.9	275.1	286.3
Canada [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	134.5		128.4	258.2	242.7
Canada [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	54.4		62.2	99.9	120.1
Canada [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2.8		3.8	6.2	7.3
Canada [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(1.8)		(2.0)	(3.3)	(3.3)
Latin America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	573.0		257.3	923.9	495.3
Latin America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	485.0		104.7	710.5	212.3
Latin America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	12.5		48.3	64.2	93.8
Latin America [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	35.1		65.0	68.7	113.3
Latin America [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	44.2		39.4	85.6	76.1
Latin America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(3.8)		(0.1)	(5.1)	(0.2)
EMEA [Member]
Disaggregation Of Revenue [Line Items]
Net sales	318.5	[1]	294.9	590.7	565.8
EMEA [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2.8	[1]	0.3	4.1	3.3
EMEA [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	304.5	[1]	277.0	564.4	529.1
EMEA [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	11.4	[1]	17.6	22.5	33.5
EMEA [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(0.2)	[1]		(0.3)	(0.1)
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	82.9		93.9	170.3	189.8
Asia Pacific [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	71.9		76.9	148.6	155.4
Asia Pacific [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 11.0		$ 17.0	$ 21.7	$ 34.4

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