Goodwill and Intangible Assets - Components of Intangible Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Finite Lived Intangible Assets [Line Items]
Weighted Average Remaining Useful Life (in Years)		9 years 1 month 6 days	9 years 10 months 24 days
Gross Carrying Amount		$ 542.9	$ 542.9
Accumulated Amortization		$ 247.0	$ 204.1
Customer Relationships
Finite Lived Intangible Assets [Line Items]
Weighted Average Remaining Useful Life (in Years)	[1],[2]	9 years 1 month 6 days	10 years
Gross Carrying Amount	[1],[2]	$ 503.8	$ 503.8
Accumulated Amortization	[1],[2]	$ 220.2	$ 180.2
Trademarks and Trade Names
Finite Lived Intangible Assets [Line Items]
Weighted Average Remaining Useful Life (in Years)		9 years 3 months 18 days	9 years 6 months
Gross Carrying Amount		$ 34.8	$ 34.8
Accumulated Amortization		$ 23.0	$ 20.6
Other Intangible Assets
Finite Lived Intangible Assets [Line Items]
Weighted Average Remaining Useful Life (in Years)		1 year 2 months 12 days	2 years 1 month 6 days
Gross Carrying Amount		$ 4.3	$ 4.3
Accumulated Amortization		$ 3.8	$ 3.3

[1] During 2019, a corrugated products facility sold part of its operations which included existing inventory, certain production equipment, and customer relationships corresponding to the operations sold. As a result, the gross carrying amount for the customer relationships intangible asset was decreased by $0.7 million

[2] During the second quarter of 2020, the Company recorded a $4.5 million adjustment to decrease the remaining book value of the customer relationships intangible asset as a result of the closure of the San Lorenzo, California corrugated products facility.