Pension and Severance Plans (Defined Benefit Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in projected benefit obligation:
Projected benefit obligation at January 1	$ 131,416	$ 109,695
Service cost	33,823	33,854	$ 12,481
Interest cost	4,067	3,641	2,954
Benefits paid	(15,183)	(8,499)
Actuarial (gain) loss	(1,387)	(2,801)
Effects of curtailment	(573)	0
Settlement	(2,496)	(1,165)
Foreign exchange (gain) loss	7,653	(3,309)
Projected benefit obligation at December 31	158,466	131,416	109,695
Change in plan assets:
Fair value of plan assets at January 1	91,471	76,042
Actual gain (loss) on plan assets	8,559	2,876
Employer contributions	28,073	25,114
Settlement	(2,496)	(1,165)
Benefits paid	(15,183)	(8,499)
Foreign exchange gain (loss)	5,301	(2,897)
Fair value of plan assets at December 31	115,725	91,471	$ 76,042
Funded status of the Plans at December 31	(42,741)	(39,945)
Defined Benefit Plan, Liability
Accumulated benefit obligation	113,600	94,500
Defined Benefit Plan, Plan with Benefit Obligation in Excess of Plan Assets [Abstract]
Aggregate projected benefit obligation	119,708	131,416
Aggregate fair value of plan assets	76,313	91,471
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Aggregate accumulated benefit obligation	53,720	49,285
Aggregate fair value of plan assets	$ 18,970	$ 16,811
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate for determining net periodic pension cost (as a percent)	3.10%	3.30%	4.20%
Discount rate for determining benefit obligations at December 31 (as a percent)	3.20%	3.10%	3.30%
Rate of compensation increase for determining net periodic pension cost (as a percent)	3.80%	3.90%	4.70%
Rate of compensation increase for determining benefit obligations at December 31 (as a percent)	3.80%	3.80%	3.90%
Expected rate of return on plan assets for determining net periodic pension cost (as a percent)	4.90%	5.00%	6.20%