Nature of Business and Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 1,459.7	$ 1,323.0
Acquisition of Businesses	12.9	140.4
Foreign Currency Effects	4.4	(3.7)
Goodwill, ending balance	1,477.0	1,459.7
Corporate/Other
Goodwill [Roll Forward]
Goodwill, beginning balance	14.4	16.0
Acquisition of Businesses	0.0	0.0
Foreign Currency Effects	0.0	(1.6)
Goodwill, ending balance	14.4	14.4
Paperboard Mills | Operating Segments
Goodwill [Roll Forward]
Goodwill, beginning balance	506.1	408.5
Acquisition of Businesses	0.0	97.6
Foreign Currency Effects	0.0	0.0
Goodwill, ending balance	506.1	506.1
Americas Paperboard Packaging | Operating Segments
Goodwill [Roll Forward]
Goodwill, beginning balance	882.0	839.0
Acquisition of Businesses	12.9	42.9
Foreign Currency Effects	1.8	0.1
Goodwill, ending balance	896.7	882.0
Europe Paperboard Packaging | Operating Segments
Goodwill [Roll Forward]
Goodwill, beginning balance	57.2	59.5
Acquisition of Businesses	0.0	(0.1)
Foreign Currency Effects	2.6	(2.2)
Goodwill, ending balance	$ 59.8	$ 57.2