Investment in Seven Hills - Additional Information (Detail) - Seven Hills - Variable Interest Entity, Not Primary Beneficiary - Equity Method Investee - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Cost of paperboard	$ 12.3	$ 15.5	$ 37.4	$ 42.3
Purchase commitments			$ 29.2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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