Capitalized Interest - Summary of Interest Costs Incurred, Capitalized, Expensed and Charged to Cost of Sales (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized Interest Costs Including Allowance for Funds Used During Construction RollForward
Interest capitalized, beginning of year	$ 5,106	$ 4,434	$ 4,072
Interest incurred	26,384	25,951	25,155
Interest expense	(24,185)	(21,707)	(23,559)
Interest charged to cost of sales	(1,722)	(3,572)	(1,234)
Interest capitalized, end of year	$ 5,583	$ 5,106	$ 4,434