Pension and Severance Plans (Impact on Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	$ 1,403,404	$ 1,217,536	$ 1,129,449
Total other comprehensive income (loss)	16,257	8,346	30,387
Balance at end of period	1,690,779	1,403,404	1,217,536
Prior Service Cost
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	582	560
Amortization included in net periodic pension cost	21	22
Net gain (loss) arising during period	0	0
Total other comprehensive income (loss)	21	22
Balance at end of period	603	582	560
Estimated amortization of cost to be included in 2018 net periodic pension cost	1
Actuarial Net Gain (Loss)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	556	(1,985)
Amortization included in net periodic pension cost	69	73
Net gain (loss) arising during period	5,075	2,468
Total other comprehensive income (loss)	5,144	2,541
Balance at end of period	5,700	556	(1,985)
Estimated amortization of cost to be included in 2018 net periodic pension cost	(147)
Total
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	1,138	(1,425)
Amortization included in net periodic pension cost	90	95
Net gain (loss) arising during period	5,075	2,468
Total other comprehensive income (loss)	5,165	2,563
Balance at end of period	6,303	$ 1,138	$ (1,425)
Estimated amortization of cost to be included in 2018 net periodic pension cost	$ (146)