Pension and Severance Plans (Impact on Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	$ 1,855,900	$ 1,719,709	$ 1,429,517
Amortization and settlement gain included in net periodic pension cost	(589)	(1,736)
Net gain (loss) arising during period	(4,108)	3,029
Total other comprehensive income (loss)	(4,697)	1,293	16,257
Balance at end of period	1,990,239	1,855,900	1,719,709
Prior Service Cost
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	602	603
Amortization and settlement gain included in net periodic pension cost	0	(1)
Net gain (loss) arising during period	0	0
Total other comprehensive income (loss)	0	(1)
Balance at end of period	602	602	603
Estimated amortization of cost to be included in 2020 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	2,057	5,700
Amortization and settlement gain included in net periodic pension cost	(589)	(1,735)
Net gain (loss) arising during period	(6,890)	(1,908)
Total other comprehensive income (loss)	(7,479)	(3,643)
Balance at end of period	(5,422)	2,057	5,700
Estimated amortization of cost to be included in 2020 net periodic pension cost	59
Total
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax [Abstract]
Balance at beginning of period	2,659	6,303
Amortization and settlement gain included in net periodic pension cost	(589)	(1,736)
Net gain (loss) arising during period	(6,890)	(1,908)
Total other comprehensive income (loss)	(7,479)	(3,644)
Balance at end of period	(4,820)	$ 2,659	$ 6,303
Estimated amortization of cost to be included in 2020 net periodic pension cost	$ 59