Segment Information - Selected Financial Information by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Net sales	$ 2,000.1	$ 1,693.7	$ 5,687.1	$ 4,944.0
Operating Income (Loss)	352.8	209.5	885.7	534.7
Non-operating pension income	5.0	0.6	14.8	1.7
Interest expense, net	(23.9)	(24.4)	(72.2)	(69.1)
Income before taxes	333.9	185.7	828.3	467.3
Trade
Segment Reporting Information [Line Items]
Net sales	2,000.1	1,693.7	5,687.1	4,944.0
Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	(40.9)	(34.3)	(113.2)	(112.9)
Operating Income (Loss)	0.0	0.0	0.0	0.0
Intersegment Eliminations | Trade
Segment Reporting Information [Line Items]
Net sales	0.0	0.0	0.0	0.0
Operating Segments
Segment Reporting Information [Line Items]
Net sales	2,000.1	1,693.7	5,687.1	4,944.0
Segment Reconciling Items
Segment Reporting Information [Line Items]
Net sales	(40.9)	(34.3)	(113.2)	(112.9)
Packaging
Segment Reporting Information [Line Items]
Net sales	1,829.4	1,501.1	5,171.4	4,378.5
Operating Income (Loss)	365.2	222.4	940.3	619.9
Packaging | Trade
Segment Reporting Information [Line Items]
Net sales	1,825.0	1,497.6	5,159.3	4,364.8
Packaging | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	4.4	3.5	12.1	13.7
Packaging | Operating Segments
Segment Reporting Information [Line Items]
Net sales	1,829.4	1,501.1	5,171.4	4,378.5
Paper
Segment Reporting Information [Line Items]
Net sales	150.3	178.1	457.1	518.7
Operating Income (Loss)	11.0	(7.3)	22.3	(21.7)
Paper | Trade
Segment Reporting Information [Line Items]
Net sales	150.3	178.1	457.0	518.7
Paper | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	0.0	0.0	0.1	0.0
Paper | Operating Segments
Segment Reporting Information [Line Items]
Net sales	150.3	178.1	457.1	518.7
Corporate and Other
Segment Reporting Information [Line Items]
Net sales	20.4	14.5	58.6	46.8
Operating Income (Loss)	(23.4)	(20.2)	(76.9)	(63.5)
Corporate and Other | Trade
Segment Reporting Information [Line Items]
Net sales	24.8	18.0	70.8	60.5
Corporate and Other | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	36.5	30.8	101.0	99.2
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales	$ 61.3	$ 48.8	$ 171.8	$ 159.7