Segment Information - Selected Financial Information by Reportable Segment (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2025	Mar. 31, 2024		Dec. 31, 2024
Segment Reporting Information [Line Items]
Net sales		$ 2,141.0	$ 1,979.5
Operating Income (Loss)	[1]	280.3	196.0
Other segment items	[1],[2]	0.0	0.0
Non-operating pension expense		0.0	1.1
Interest expense, net		(12.9)	(9.6)
Income before taxes		267.4	187.5
Depreciation, amortization, and depletion		138.0	128.4
Capital expenditures	[3]	148.1	76.7
Assets		8,969.8	8,857.0		$ 8,833.2
Variable Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[4]	0.0	0.0
Fixed Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[5]	0.0	0.0
Freight
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1]	0.0	0.0
Operating Segments
Segment Reporting Information [Line Items]
Net sales		2,180.9	2,023.7
Operating Segments | Trade
Segment Reporting Information [Line Items]
Net sales		2,141.0	1,979.5
Operating Segments | Intersegment
Segment Reporting Information [Line Items]
Net sales		39.9	44.2
Elimination of intersegment sales
Segment Reporting Information [Line Items]
Net sales		(39.9)	(44.2)
Packaging
Segment Reporting Information [Line Items]
Net sales		1,970.3	1,798.3
Operating Income (Loss)	[1]	278.1	203.8
Other segment items	[1],[2]	(86.8)	(62.9)
Depreciation, amortization, and depletion		128.4	118.5
Capital expenditures	[3]	131.1	71.7
Assets		7,349.3	6,888.5
Packaging | Variable Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[4]	(961.3)	(939.5)
Packaging | Fixed Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[5]	(447.5)	(405.0)
Packaging | Trade
Segment Reporting Information [Line Items]
Net sales		1,970.3	1,793.5
Packaging | Intersegment
Segment Reporting Information [Line Items]
Net sales		0.0	4.8
Packaging | Freight
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1]	(196.6)	(187.1)
Packaging | Operating Segments
Segment Reporting Information [Line Items]
Net sales		1,970.3	1,798.3
Paper
Segment Reporting Information [Line Items]
Net sales		154.2	163.8
Operating Income (Loss)	[1]	35.6	29.7
Other segment items	[1],[2]	(0.1)	(5.2)
Depreciation, amortization, and depletion		4.6	6.0
Capital expenditures	[3]	0.8	1.3
Assets		398.1	394.7
Paper | Variable Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[4]	(82.8)	(89.3)
Paper | Fixed Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[5]	(18.8)	(21.5)
Paper | Trade
Segment Reporting Information [Line Items]
Net sales		154.2	163.8
Paper | Intersegment
Segment Reporting Information [Line Items]
Net sales			0.0
Paper | Freight
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1]	(16.9)	(18.1)
Paper | Operating Segments
Segment Reporting Information [Line Items]
Net sales		154.2	163.8
Corporate and Other
Segment Reporting Information [Line Items]
Net sales		16.5	17.4	[6]
Operating Income (Loss)	[1]	(33.4)	(37.5)	[7]
Other segment items	[1],[2]	(89.8)	(99.1)
Depreciation, amortization, and depletion		5.0	3.9
Capital expenditures	[3]	16.2	3.7
Assets		1,222.4	1,573.8
Corporate and Other | Variable Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[4]	0.0	0.0
Corporate and Other | Fixed Costs [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1],[5]	0.0	0.0
Corporate and Other | Trade
Segment Reporting Information [Line Items]
Net sales		16.5	22.2
Corporate and Other | Intersegment
Segment Reporting Information [Line Items]
Net sales		39.9	39.4
Corporate and Other | Freight
Segment Reporting Information [Line Items]
Operating Income (Loss)	[1]	0.0	0.0
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales		$ 56.4	$ 61.6

[1] The significant expense categories align with the segment-level information that is regularly provided to the CODM.

[2] Other segment items for each reportable segment primarily include: 1. Packaging: farmout purchases, certain divisional allocations, and other expense/income items. 2. Paper: other expense/income items. 3. Corporate and Other: unallocated corporate costs, transportation business activity, and activity related to LTP.

[3] Includes “Additions to property, plant, and equipment” and excludes cash used for “Acquisition of business, net of cash acquired” as reported on our Consolidated Statements of Cash Flows.

[4] For the Packaging segment, primarily includes expense items for liner and medium consumption, purchased sheets usage, liner board discount, raw materials, and hourly-employee related expenses and benefits. For the Paper segment, primarily includes expense items for chemicals, raw materials, finishing materials, and hourly-employee related expenses and benefits.

[5] For the Packaging segment, primarily includes expense items for depreciation and salaried employee-related expenses and benefits. For the Paper segment, primarily includes expense items for depreciation, salaried employee-related expenses, and professional services.

[6] The Corporate and Other segment sales to external customers is presented net of total company intersegment eliminations.

[7] The significant expense categories reported for the Packaging and Paper segments are not used for Corporate and Other in the segment-level information that is regularly reviewed by the CODM. The CODM makes resource allocation decisions for Corporate and Other based on divisional income (loss) from operations.