Pension and Severance Plans (Defined Benefit Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in projected benefit obligation:
Projected benefit obligation at January 1	$ 175,712	$ 158,466
Service cost	31,355	32,913	$ 33,823
Interest cost	5,244	4,867	4,067
Benefits paid	(11,742)	(6,137)
Actuarial (gain) loss	12,540	(5,991)
Settlement	(5,310)	(5,055)
Foreign exchange (gain) loss	(351)	(3,351)
Projected benefit obligation at December 31	207,448	175,712	158,466
Change in plan assets:
Fair value of plan assets at January 1	123,370	115,725
Actual gain (loss) on plan assets	12,221	(4,210)
Employer contributions	27,134	26,899
Settlement	(5,310)	(5,055)
Benefits paid	(11,742)	(6,137)
Foreign exchange gain (loss)	(515)	(3,852)
Fair value of plan assets at December 31	145,158	123,370	$ 115,725
Funded status of the Plans at December 31	(62,290)	(52,342)
Defined Benefit Plan, Liability
Accumulated benefit obligation	154,700	132,900
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Aggregate projected benefit obligation	197,377	135,967
Aggregate fair value of plan assets	134,589	80,885
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Aggregate accumulated benefit obligation	71,059	63,274
Aggregate fair value of plan assets	$ 24,043	$ 20,094
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%	3.20%	3.10%
Discount rate for determining benefit obligations at December 31 (as a percent)	2.50%	3.10%	3.20%
Rate of compensation increase for determining net periodic pension cost (as a percent)	3.60%	3.80%	3.80%
Rate of compensation increase for determining benefit obligations at December 31 (as a percent)	3.70%	3.60%	3.80%
Expected rate of return on plan assets for determining net periodic pension cost (as a percent)	5.10%	4.90%	4.90%