Plant, Equipment and Timberlands - Summary of Interest Expense Capitalized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Expense [Abstract]
Interest cost incurred	$ 19,852	$ 17,431	$ 17,815
Interest capitalized	2,080	1,609	351
Interest expense	$ 17,772	$ 15,822	$ 17,464