Accrued Severance Benefits - Changes in Accrued Severance Benefits (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Postemployment Benefits [Abstract]
Beginning balance	$ 152,780	$ 140,878	$ 149,795	$ 130,144
Provisions	4,653	3,390	9,165	10,776
Severance payments	(3,507)	(10,558)	(5,754)	(18,082)
Translation adjustments	(7,551)	(2,816)	(6,831)	8,056
Ending balance	146,375	130,894	146,375	130,894
Less: Cumulative contributions to the National Pension Fund	(236)	(245)	(236)	(245)
Group severance insurance plan	(597)	(599)	(597)	(599)
Accrued severance benefits, net	$ 145,542	$ 130,050	$ 145,542	$ 130,050	$ 148,905