Pension and Severance Plans (Defined Benefit Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in projected benefit obligation:
Projected benefit obligation at January 1	$ 207,448	$ 175,712
Service cost	29,848	31,355	$ 32,913
Interest cost	4,980	5,244	4,867
Benefits paid	(8,380)	(11,742)
Actuarial (gain) loss	6,104	12,540
Effects of curtailment	(2,410)	0
Settlement (1)	(31,804)	(5,310)
Foreign exchange (gain) loss	16,723	(351)
Projected benefit obligation at December 31	222,509	207,448	175,712
Change in plan assets:
Fair value of plan assets at January 1	145,158	123,370
Actual gain (loss) on plan assets	11,400	12,221
Employer contributions	25,080	27,134
Settlement (1)	(31,804)	(5,310)
Benefits paid	(8,380)	(11,742)
Foreign exchange gain (loss)	13,757	(515)
Fair value of plan assets at December 31	155,211	145,158	$ 123,370
Funded status of the Plans at December 31	(67,298)	(62,290)
Defined Benefit Plan, Liability
Accumulated benefit obligation	164,700	154,700
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Aggregate projected benefit obligation	140,424	197,377
Aggregate fair value of plan assets	68,270	134,589
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Aggregate accumulated benefit obligation	76,426	71,059
Aggregate fair value of plan assets	$ 25,292	$ 24,043
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)	2.50%	3.10%	3.20%
Discount rate for determining benefit obligations at December 31 (as a percent)	2.30%	2.50%	3.10%
Rate of compensation increase for determining net periodic pension cost (as a percent)	3.70%	3.60%	3.80%
Rate of compensation increase for determining benefit obligations at December 31 (as a percent)	3.70%	3.70%	3.60%
Expected rate of return on plan assets for determining net periodic pension cost (as a percent)	3.80%	5.10%	4.90%