Pension and Severance Plans (Korean Severance Plan) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Liability, Current and Noncurrent [Roll Forward]
Total accrued severance plan obligations at December 31	$ 142,126	$ 153,735	$ 136,214
Less current portion of accrued severance plan obligations (Note 10)	13,179	15,190	14,053
Non-current portion of accrued severance plan obligations	128,947	138,545	122,161
Korean Severance Plan
Deferred Compensation Liability, Current and Noncurrent [Roll Forward]
Balance at January 1	153,920	136,396	143,151
Provision of severance benefits	1,939	11,714	6,746
Severance payments	(7,611)	(11,787)	(9,429)
Foreign currency (gain) loss	(5,950)	17,597	(4,072)
Balance at the end of year	142,298	153,920	136,396
Payments remaining with the National Pension Fund	$ (172)	$ (185)	$ (182)