Geographic and Segment Information - Net Sales by Region, Based on Location of Products are Billed (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	$ 174,580	$ 176,697	$ 166,685	$ 161,710	$ 180,462	$ 192,296	$ 167,106	$ 148,105	$ 679,672	$ 687,969	$ 633,712
Korea [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales									279,883	219,618	241,715
Asia Pacific (Other Than Korea) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales									322,595	391,875	316,562
U.S.A. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales									35,089	33,201	51,164
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales									41,109	42,274	23,461
Others [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales									$ 996	$ 1,001	$ 810