Segment Information - Selected Financial Information by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2024		Sep. 30, 2023		Sep. 30, 2024		Sep. 30, 2023
Segment Reporting Information [Line Items]
Net sales	$ 2,182.4		$ 1,936.0		$ 6,237.2		$ 5,864.5
Operating Income (Loss)	327.1		258.6		799.1		813.6
Non-operating pension expense	1.2		(1.8)		3.4		(5.8)
Interest expense, net	(9.7)		(12.3)		(29.7)		(42.2)
Income before taxes	318.6		244.5		772.8		765.6
Trade
Segment Reporting Information [Line Items]
Net sales	2,182.4		1,936.0		6,237.2		5,864.5
Operating Segments
Segment Reporting Information [Line Items]
Net sales	2,182.4		1,936.0		6,237.2		5,864.5
Material Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net sales	(51.2)		(45.2)		(140.5)		(130.9)
Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	(51.2)		(45.2)		(140.5)		(130.9)
Packaging
Segment Reporting Information [Line Items]
Net sales	2,008.7		1,759.8		5,715.3		5,358.7
Operating Income (Loss)	320.7	[1]	256.8	[2]	804.3	[1],[3]	810.5	[2]
Packaging | Trade
Segment Reporting Information [Line Items]
Net sales	1,998.8	[1]	1,755.2	[2]	5,694.3	[1],[3]	5,344.3	[2]
Packaging | Intersegment
Segment Reporting Information [Line Items]
Net sales	9.9	[1]	4.6	[2]	21.0	[1],[3]	14.4	[2]
Packaging | Operating Segments
Segment Reporting Information [Line Items]
Net sales	2,008.7	[1]	1,759.8	[2]	5,715.3	[1],[3]	5,358.7	[2]
Paper
Segment Reporting Information [Line Items]
Net sales	159.3		157.9		473.2		451.6
Operating Income (Loss)	38.5		27.6	[2]	94.9	[3]	90.8	[2]
Paper | Trade
Segment Reporting Information [Line Items]
Net sales	159.3		157.9	[2]	473.2	[3]	451.6	[2]
Paper | Operating Segments
Segment Reporting Information [Line Items]
Net sales	159.3		157.9	[2]	473.2	[3]	451.6	[2]
Corporate and Other
Segment Reporting Information [Line Items]
Net sales	14.4		18.3		48.7		54.2
Operating Income (Loss)	(32.1)		(25.8)		(100.1)		(87.7)
Corporate and Other | Trade
Segment Reporting Information [Line Items]
Net sales	24.3		22.9		69.7		68.6
Corporate and Other | Intersegment
Segment Reporting Information [Line Items]
Net sales	41.3		40.6		119.5		116.5
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales	$ 65.6		$ 63.5		$ 189.2		$ 185.1

[1] The three and nine months ended September 30, 2024 include $0.9 million and $1.0 million, respectively, of charges consisting of closure costs related to corrugated products facilities. For the nine months ended September 30, 2024, these charges were partially offset by income primarily related to a favorable lease buyout for a closed corrugated products facility during the first quarter of 2024.

[2] The three and nine months ended September 30, 2023 include the following: 1. $2.6 million and $8.2 million, respectively, of charges related to the announced discontinuation of production of UFS paper grades on the No. 3 machine at the Jackson, Alabama mill associated with the permanent conversion of the machine to produce linerboard and other paper-to-containerboard conversion related activities. 2. $0.1 million of income and $13.5 million of charges, respectively, related to the closure of corrugated products facilities and design centers. Included therein are closure costs as well as the gain on sale of a corrugated products facility.

[3] The nine months ended September 30, 2024 include $9.7 million of charges related to the announced discontinuation of production of UFS paper grades on the No. 3 machine at the Jackson, Alabama mill associated with the permanent conversion of the machine to produce linerboard and other paper-to-containerboard conversion related activities.