Other Expense, Net - Components of Other Income (Expense), Net (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset disposals and write-offs		$ (40.8)	$ (39.7)	$ (31.7)
Facilities closure and other costs	[1]	19.4	(1.0)	(7.9)
DeRidder and other litigation	[2]	(3.5)	(95.2)	0.0
DeRidder and other litigation insurance recoveries	[2]	3.5	95.2	0.0
Wallula mill restructuring	[3]	(87.0)	0.0	0.0
Acquisition and integration-related costs	[4]	13.3	0.0	0.0
Jackson mill conversion-related activities	[5]	0.0	(7.6)	(1.8)
Other		(26.7)	(23.2)	(1.5)
Total		$ (148.4)	$ (71.5)	$ (42.9)

[1] For 2025, includes income consisting of gains on sales of closed corrugated products facilities and a gain on an asset disposal related to a closed corrugated products facility, partially offset by charges related to the closure of certain corrugated products facilities. For 2024, includes charges consisting of closure costs related to corrugated products facilities, partially offset by income primarily related to a favorable lease buyout for a closed corrugated products facility. For 2023, includes charges consisting of closure costs related to corrugated products facilities and design centers, partially offset by a gain on sale of a corrugated products facility.

[2] For 2025, includes charges and income related to fully insured settlement amounts, and for 2024, includes charges and income related to the DeRidder mill lawsuit for compensatory damages plus interest. For more information, see Note 20, Commitments, Guarantees, Indemnifications, and Legal Proceedings.

[3] Includes charges related to the announced discontinuation of the No. 2 machine and kraft pulping facilities at the Wallula, Washington containerboard mill.

[4] Includes charges for acquisition and integration costs related to the September 2025 Greif Acquisition.

[5] Includes charges related to the announced discontinuation of production of uncoated freesheet paper grades on the No. 3 machine at the Jackson, Alabama mill in the first quarter of 2021 associated with the permanent conversion of the machine to produce linerboard and other paper-to-containerboard conversion related activities.