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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Business Segment and Geographic Area Information - Segment Reporting, by Segment (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) paperboard_mill	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment paperboard_mill	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of segments \| segment									3
Number of paperboard mill facilities \| paperboard_mill	9								9
Net sales									$ 6,140.8	$ 6,011.9	$ 4,384.9
Net Sales	$ 1,519.8	$ 1,581.6	$ 1,552.8	$ 1,505.9	$ 1,509.3	$ 1,531.8	$ 1,510.9	$ 1,477.4	6,160.1	6,029.4	4,405.6
Income from Operations	132.1	$ 122.9	$ 144.4	$ 134.2	107.6	$ 166.6	$ 110.1	$ 74.2	533.6	458.5	327.9
Capital Expenditures									352.9	395.2	260.1
Depreciation and Amortization									447.2	430.6	330.3
Assets	7,276.7				7,048.8				7,276.7	7,048.8	4,821.3
Operating Segments \| Paperboard Mills
Segment Reporting Information [Line Items]
Net sales									1,094.8	1,078.1	399.7
Income from Operations									33.1	30.6	(35.0)
Capital Expenditures									208.0	240.1	111.4
Depreciation and Amortization									224.4	197.5	143.7
Assets	2,912.2				3,005.6				2,912.2	3,005.6	1,487.0
Operating Segments \| Americas Paperboard Packaging
Segment Reporting Information [Line Items]
Net sales									4,233.7	4,098.3	3,245.1
Income from Operations									477.7	420.1	358.2
Capital Expenditures									94.7	104.3	98.8
Depreciation and Amortization									165.1	165.4	125.3
Assets	3,392.3				3,143.6				3,392.3	3,143.6	2,478.7
Operating Segments \| Europe Paperboard Packaging
Segment Reporting Information [Line Items]
Net sales									689.3	695.9	593.5
Income from Operations									60.3	46.1	37.3
Capital Expenditures									34.5	19.5	17.3
Depreciation and Amortization									36.7	48.9	42.1
Assets	686.3				603.4				686.3	603.4	607.1
Corporate/Other
Segment Reporting Information [Line Items]
Net Sales									142.3	157.1	167.3
Income from Operations									(37.5)	(38.3)	(32.6)
Capital Expenditures									15.7	31.3	32.6
Depreciation and Amortization									21.0	18.8	19.2
Assets	$ 285.9				$ 296.2				$ 285.9	$ 296.2	$ 248.5