Investment in Seven Hills - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Capital contributions to equity method investment	$ (2,219)	$ (349)	$ (103)
Estimated redemption value of voint venture	8,900	8,300
Seven Hills | Variable Interest Entity, Not Primary Beneficiary | Equity Method Investee
Schedule of Equity Method Investments [Line Items]
Cost of paperboard	55,800	$ 47,000	$ 45,500
Purchase commitments	$ 26,300