Pension and Severance Plans (Defined Benefit Plans - Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan [Abstract]
Prepaid benefit cost (included in non-current assets)	$ 656	$ 0
Accrued benefit liability (included in pension and severance obligations)	(43,397)	(39,945)
Net amount recognized at year end	$ (42,741)	$ (39,945)