Segment Information - Selected Financial Information by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019
Segment Reporting Information [Line Items]
Net sales	$ 1,708.7		$ 1,733.7
Operating Income (Loss)	209.1		275.4
Non-operating pension income and expense	0.6		(2.0)
Interest expense, net	(19.6)		(24.1)
Income before taxes	190.1		249.3
Trade
Segment Reporting Information [Line Items]
Net sales	1,708.7		1,733.7
Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	(36.4)		(40.3)
Operating Segments
Segment Reporting Information [Line Items]
Net sales			1,733.7
Segment Reconciling Items
Segment Reporting Information [Line Items]
Net sales	(36.4)		(40.3)
Packaging
Segment Reporting Information [Line Items]
Net sales	1,467.5		1,477.6
Operating Income (Loss)	199.8	[1]	249.6	[2]
Packaging | Trade
Segment Reporting Information [Line Items]
Net sales	1,465.4		1,470.8
Packaging | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	2.1		6.8
Packaging | Operating Segments
Segment Reporting Information [Line Items]
Net sales	1,467.5		1,477.6
Paper
Segment Reporting Information [Line Items]
Net sales	217.4		239.7
Operating Income (Loss)	32.5	[1]	45.6	[2]
Paper | Trade
Segment Reporting Information [Line Items]
Net sales	217.4		239.7
Paper | Operating Segments
Segment Reporting Information [Line Items]
Net sales	217.4		239.7
Corporate and Other
Segment Reporting Information [Line Items]
Net sales	23.8		16.4
Operating Income (Loss)	(23.2)		(19.8)
Corporate and Other | Trade
Segment Reporting Information [Line Items]
Net sales	25.9		23.2
Corporate and Other | Intersegment Eliminations
Segment Reporting Information [Line Items]
Net sales	34.3		33.5
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales	$ 60.2		$ 56.7

[1] (a) 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.   2. $0.4 million of charges consisting of closure costs related to corrugated products facilities.

[2] Includes $0.6 million of charges related to the second quarter 2018 discontinuation of uncoated free sheet and coated one-side grades at the Wallula, Washington mill associated with the conversion of the No. 3 paper machine to produce virgin kraft linerboard.