Divestitures and Impairments of Businesses (Narrative) (Details) € in Millions, R$ in Millions, $ in Millions	3 Months Ended						12 Months Ended
	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 BRL (R$)	Dec. 31, 2020 EUR (€)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net (gains) losses on sales and impairments of businesses							$ 465.0			$ 205.0	$ 122.0
Printing Papers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of ownership retained							19.90%	19.90%	19.90%
Olmuksan [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net (gains) losses on sales and impairments of businesses	$ 123.0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Brazilian Industrial Packaging
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses							$ 58.5	R$ 330.0
Proceeds to be received at closing							49.6	280.0
Deferred proceeds on disposal of business							$ 8.9	R$ 50.0
Number of containerboard mill to be sold							3	3	3
Number of box plants to be sold							4	4	4
Net (gains) losses on sales and impairments of businesses							$ 347.0				122.0
Net gains (losses) on sales and impairments of businesses, net of tax							340.0				$ 81.0
Other comprehensive income (loss), foreign currency transaction and translation adjustment, before tax							327.0
Impairment of long-lived assets to be disposed of							20.0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Olmuksan [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses							$ 81.0		€ 66.0
Net (gains) losses on sales and impairments of businesses	123.0
Net gains (losses) on sales and impairments of businesses, net of tax	(123.0)
Sale of Stock, Percentage of Ownership before Transaction							90.38%	90.38%	90.38%
Assets, Fair Value Disclosure	$ 79.0						$ 79.0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | APPM [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses										82.0
Net (gains) losses on sales and impairments of businesses		$ (6.0)	$ 17.0	$ (1.0)	$ 8.0	$ 152.0				159.0
Net gains (losses) on sales and impairments of businesses, net of tax										$ 157.0
Gain (loss) on sale of assets and impairment charge allocated to minority interest					$ 2.0	$ 7.0	9.0
Gain (loss) on sale of assets and impairment charge allocated to minority interest							$ 9.0