Segment Information - Selected Financial Information by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2024		Jun. 30, 2023		Jun. 30, 2024		Jun. 30, 2023
Segment Reporting Information [Line Items]
Net sales	$ 2,075.3		$ 1,952.1		$ 4,054.8		$ 3,928.4
Operating Income (Loss)	276.0		284.4		472.0		555.0
Non-operating pension expense	1.1		(2.0)		2.2		(4.0)
Interest expense, net	(10.4)		(14.6)		(19.9)		(29.9)
Income before taxes	266.7		267.8		454.3		521.1
Trade
Segment Reporting Information [Line Items]
Net sales	2,075.3		1,952.1		4,054.8		3,928.4
Operating Segments
Segment Reporting Information [Line Items]
Net sales	2,075.3		1,952.1		4,054.8		3,928.4
Material Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net sales	(45.1)		(42.0)		(89.3)		(85.8)
Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	(45.1)		(42.0)		(89.3)		(85.8)
Packaging
Segment Reporting Information [Line Items]
Net sales	1,908.3		1,790.3		3,706.5		3,598.9
Operating Income (Loss)	279.8	[1]	285.8	[2]	483.6	[1]	553.7	[2]
Packaging | Trade
Segment Reporting Information [Line Items]
Net sales	1,902.0	[1]	1,785.7	[2]	3,695.4	[1]	3,589.1	[2]
Packaging | Intersegment
Segment Reporting Information [Line Items]
Net sales	6.3	[1]	4.6	[2]	11.1	[1]	9.8	[2]
Packaging | Operating Segments
Segment Reporting Information [Line Items]
Net sales	1,908.3	[1]	1,790.3	[2]	3,706.5	[1]	3,598.9	[2]
Paper
Segment Reporting Information [Line Items]
Net sales	150.1		142.8		313.9		293.7
Operating Income (Loss)	26.7	[1]	29.1	[2]	56.4	[1]	63.2	[2]
Paper | Trade
Segment Reporting Information [Line Items]
Net sales	150.1	[1]	142.8	[2]	313.9	[1]	293.7	[2]
Paper | Operating Segments
Segment Reporting Information [Line Items]
Net sales	150.1	[1]	142.8	[2]	313.9	[1]	293.7	[2]
Corporate and Other
Segment Reporting Information [Line Items]
Net sales	16.9		19.0		34.4		35.8
Operating Income (Loss)	(30.5)		(30.5)		(68.0)		(61.9)
Corporate and Other | Trade
Segment Reporting Information [Line Items]
Net sales	23.2		23.6		45.5		45.6
Corporate and Other | Intersegment
Segment Reporting Information [Line Items]
Net sales	38.8		37.4		78.2		76.0
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales	$ 62.0		$ 61.0		$ 123.7		$ 121.6

[1] The three and six months ended June 30, 2024 include the following: 1. $0.6 million of income and $9.7 million of charges, respectively, 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 charges consisting of closure costs related to corrugated products facilities. For the six months ended June 30, 2024, these charges were completely offset by $0.1 million of income primarily related to a favorable lease buyout for a closed corrugated products facility during the first quarter of 2024.

[2] The three and six months ended June 30, 2023 include the following: 1. $4.4 million and $5.7 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. $3.9 million and $13.6 million, respectively, of charges consisting of closure costs related to corrugated products facilities and design centers.