Accrued Severance Benefits - Changes in Accrued Severance Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postemployment Benefits [Abstract]
Beginning balance	$ 130,144	$ 135,160
Provisions	24,373	14,432	$ 15,289
Severance payments	(21,506)	(15,352)	(11,394)
Translation adjustments	16,785	(4,096)
Ending balance	149,796	130,144	$ 135,160
Less: Cumulative contributions to the National Pension Fund	(259)	(276)
Group severance insurance plan	(632)	(643)
Accrued severance benefits, net	$ 148,905	$ 129,225