Pension and Severance Plans (Impact on Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	$ 1,719,709	$ 1,429,517	$ 1,217,536
Total other comprehensive income (loss)	1,293	16,257	8,346
Balance at end of period	1,855,900	1,719,709	1,429,517
Prior Service Cost
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	603	582
Amortization and settlement gain included in net periodic pension cost	(1)	21
Net gain (loss) arising during period	0	0
Total other comprehensive income (loss)	(1)	21
Balance at end of period	602	603	582
Estimated amortization of cost to be included in 2019 net periodic pension cost	0
Actuarial Net Gain (Loss)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	5,700	556
Amortization and settlement gain included in net periodic pension cost	(1,735)	69
Net gain (loss) arising during period	(1,908)	5,075
Total other comprehensive income (loss)	(3,643)	5,144
Balance at end of period	2,057	5,700	556
Estimated amortization of cost to be included in 2019 net periodic pension cost	(374)
Total
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	6,303	1,138
Amortization and settlement gain included in net periodic pension cost	(1,736)	90
Net gain (loss) arising during period	(1,908)	5,075
Total other comprehensive income (loss)	(3,644)	5,165
Balance at end of period	2,659	$ 6,303	$ 1,138
Estimated amortization of cost to be included in 2019 net periodic pension cost	$ (374)