Geographic and Segment Information - Net Sales by Region, Based on Location of Products are Billed (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	$ 199,685	$ 166,685	$ 365,504	$ 328,395
Korea [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	58,060	67,979	112,750	133,062
Asia Pacific (Other Than Korea) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	119,725	76,524	209,430	155,988
U.S.A. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	13,593	11,155	23,411	20,322
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	7,619	10,725	18,597	18,578
Others [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	$ 688	$ 302	$ 1,316	$ 445