Pension and Severance Plans (Changes and Components) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan [Abstract]
Service cost	$ 33,823	$ 33,854	$ 12,481
Interest cost	4,067	3,641	2,954
Expected return on plan assets	(4,537)	(3,788)	(3,330)
Amortization of prior service cost	30	35	34
Recognized actuarial (gain) loss	84	94	91
Net periodic pension cost	33,467	33,836	12,230
Curtailment loss	574	0	0
Settlement (gain) loss	383	128	27
Total pension expense	$ 34,424	$ 33,964	$ 12,257