Pension and Severance Plans (Korean Severance Plan) (Details) - Korean Severance Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Compensation Liability, Current and Noncurrent [Roll Forward]
Balance at January 1	$ 142,298	$ 153,920	$ 136,396
Provision of severance benefits	1,306	1,939	11,714
Severance payments	(10,659)	(7,611)	(11,787)
Foreign currency (gain) loss	(5,398)	(5,950)	17,597
Balance at the end of year	127,547	142,298	153,920
Payments remaining with the National Pension Fund	(161)	(172)	(185)
Total accrued severance plan obligations at December 31	127,386	142,126	153,735
Less current portion of accrued severance plan obligations (Note 11)	13,408	13,179	15,190
Non-current portion of accrued severance plan obligations	$ 113,978	$ 128,947	$ 138,545