Segment Information - Selected Financial Information by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021		Mar. 31, 2020
Segment Reporting Information [Line Items]
Net sales	$ 1,807.2		$ 1,708.7
Operating Income (Loss)	238.3		209.1
Non-operating pension income and expense	4.8		0.6
Interest expense, net	(23.5)		(19.6)
Income before taxes	219.6		190.1
Trade
Segment Reporting Information [Line Items]
Net sales	1,807.2		1,708.7
Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	(36.4)		(36.4)
Operating Segments
Segment Reporting Information [Line Items]
Net sales	1,807.2		1,708.7
Segment Reconciling Items
Segment Reporting Information [Line Items]
Net sales	(36.4)		(36.4)
Packaging
Segment Reporting Information [Line Items]
Net sales	1,623.6		1,467.5
Operating Income (Loss)	257.9	[1]	199.8	[2]
Packaging | Trade
Segment Reporting Information [Line Items]
Net sales	1,619.8		1,465.4
Packaging | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	3.8		2.1
Packaging | Operating Segments
Segment Reporting Information [Line Items]
Net sales	1,623.6		1,467.5
Paper
Segment Reporting Information [Line Items]
Net sales	164.6		217.4
Operating Income (Loss)	8.7	[1]	32.5	[2]
Paper | Trade
Segment Reporting Information [Line Items]
Net sales	164.4		217.4
Paper | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	0.2
Paper | Operating Segments
Segment Reporting Information [Line Items]
Net sales	164.6		217.4
Corporate and Other
Segment Reporting Information [Line Items]
Net sales	19.0		23.8
Operating Income (Loss)	(28.3)		(23.2)
Corporate and Other | Trade
Segment Reporting Information [Line Items]
Net sales	23.0		25.9
Corporate and Other | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	32.4		34.3
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales	$ 55.4		$ 60.2

[1] The three months ended March 31, 2021 include the following:

1. $2.1 million of charges consisting of closure costs related to corrugated products facilities.

2. $1.1 million of 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.

[2] The three months ended March 31, 2020 include the following:

1.$0.8 million of incremental, out-of-pocket costs related to COVID-19, including supplies, cleaning and sick pay. Beginning in July 2020, all corresponding COVID-19 related expenses were included in normalized costs.

2.$0.4 million of charges consisting of closure costs related to corrugated products facilities.