Revenue Recognition - Schedule of Disaggregates Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2024		Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Disaggregation Of Revenue [Line Items]
Net sales	$ 4,726.7		$ 5,277.6	$ 9,346.7	$ 10,200.7
Operating Segments [Member]
Disaggregation Of Revenue [Line Items]
Net sales	4,800.0		5,368.0	9,487.2	10,365.5
Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,398.3		2,627.4	4,818.2	4,964.8
Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,113.5		1,265.1	2,172.8	2,480.1
Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,016.2		1,168.2	1,934.5	2,291.8
Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	272.0		307.3	561.7	628.8
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(73.3)		(90.4)	(140.5)	(164.8)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(62.9)		(81.6)	(122.1)	(150.0)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(8.4)		(7.5)	(13.7)	(12.2)
Intersegment Sales [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(2.0)		(1.3)	(4.7)	(2.6)
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,467.4		3,975.3	6,865.2	7,879.7
U.S. [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,773.2		2,001.6	3,569.6	3,975.1
U.S. [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	629.7		741.7	1,223.5	1,444.7
U.S. [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	901.3		1,056.3	1,721.8	2,079.0
U.S. [Member] | Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	229.0		260.3	477.1	537.0
U.S. [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(65.8)		(84.6)	(126.8)	(156.1)
Latin America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	561.3		573.0	1,112.6	923.9
Latin America [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	496.1		485.0	981.5	710.5
Latin America [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	0.5		12.5	6.6	64.2
Latin America [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	26.7		35.1	50.2	68.7
Latin America [Member] | Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	42.0		44.2	82.4	85.6
Latin America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(4.0)		(3.8)	(8.1)	(5.1)
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	317.8		327.9	621.2	636.1
Canada [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	127.4		138.0	262.5	275.1
Canada [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	126.4		134.5	243.1	258.2
Canada [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	63.9		54.4	115.0	99.9
Canada [Member] | Operating Segments [Member] | Distribution Segment [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1.0		2.8	2.2	6.2
Canada [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(0.9)		(1.8)	(1.6)	(3.3)
EMEA [Member]
Disaggregation Of Revenue [Line Items]
Net sales	297.9	[1]	318.5	582.3	590.7
EMEA [Member] | Operating Segments [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1.6	[1]	2.8	4.6	4.1
EMEA [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	286.8	[1]	304.5	557.7	564.4
EMEA [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	12.1	[1]	11.4	24.0	22.5
EMEA [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(2.6)	[1]	(0.2)	(4.0)	(0.3)
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	82.3		82.9	165.4	170.3
Asia Pacific [Member] | Operating Segments [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	70.1		71.9	141.9	148.6
Asia Pacific [Member] | Operating Segments [Member] | Global Paper [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 12.2		$ 11.0	$ 23.5	$ 21.7

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