DIVESTITURES Narrative (Details) € in Millions, R$ in Millions, $ in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2021 EUR (€)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 BRL (R$)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net (gains) losses on sales and impairments of businesses	$ 2.0			$ 344.0
Change in cumulative foreign currency translation adjustment	143.0			544.0
Kwidzyn Mill
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses	$ 786.0	€ 670.0
Printing Papers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of Stock, Percentage of Ownership after Transaction	19.90%	19.90%
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Olmuksan
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses	$ 78.0	€ 66.0
Sale of stock, percentage of ownership before transaction	90.38%	90.38%
Assets, Fair Value Disclosure			$ 79.0		$ 79.0
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
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax	$ 2.0
Change in cumulative foreign currency translation adjustment	$ 2.0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Brazilian Industrial Packaging [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses					58.5	R$ 330
Deferred proceeds on disposal of business					8.9	50
Net (gains) losses on sales and impairments of businesses				347.0
Net (gains) losses on sales and impairments of businesses, net of tax				340.0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax				(327.0)
Proceeds to be received at closing					$ 49.6	R$ 280
Number of containerboard mill to be sold					3	3
Number of box plants to be sold					4	4
Impairment of Long-Lived Assets to be Disposed of				$ 20.0