Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Capital Stock [Member]	Surplus [Member]	Undivided Profits [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2018	$ 100,175	$ 176,710	$ 256,397	$ (10,309)	$ (33,102)	$ 489,871
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	14,558	0	0	14,558
Other comprehensive income, net of tax	0	0	0	3,298	0	3,298
Stock options exercised	5	30	0	0	0	35
Cash dividend declared	0	0	(6,591)	0	0	(6,591)
Purchase of treasury stock	0	0	0	0	(35)	(35)
Sale of treasury stock	0	(218)	0	0	812	594
Stock based compensation expense	0	(12)	0	0	0	(12)
Balance at Mar. 31, 2019	100,180	176,510	264,364	(7,011)	(32,325)	501,718
Balance at Dec. 31, 2018	100,175	176,710	256,397	(10,309)	(33,102)	489,871
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income						43,933
Other comprehensive income, net of tax						10,238
Balance at Sep. 30, 2019	100,200	176,395	280,542	(71)	(30,903)	526,163
Balance at Mar. 31, 2019	100,180	176,510	264,364	(7,011)	(32,325)	501,718
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	14,667	0	0	14,667
Other comprehensive income, net of tax	0	0	0	5,237	0	5,237
Cash dividend declared	0	0	(6,598)	0	0	(6,598)
Sale of treasury stock	0	(120)	0	0	720	600
Stock based compensation expense	0	6	0	0	0	6
Balance at Jun. 30, 2019	100,180	176,396	272,433	(1,774)	(31,605)	515,630
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	14,708	0	0	14,708
Other comprehensive income, net of tax	0	0	0	1,703	0	1,703
Stock options exercised	20	98	0	0	0	118
Cash dividend declared	0	0	(6,599)	0	0	(6,599)
Sale of treasury stock	0	(105)	0	0	702	597
Stock based compensation expense	0	6	0	0	0	6
Balance at Sep. 30, 2019	100,200	176,395	280,542	(71)	(30,903)	526,163
Balance at Dec. 31, 2019	100,205	176,427	288,067	4,461	(30,903)	538,257
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	13,313	0	0	13,313
Other comprehensive income, net of tax	0	0	0	6,931	0	6,931
Cash dividend declared	0	0	(6,827)	0	0	(6,827)
Purchase of treasury stock	0	0	0	0	(3,493)	(3,493)
Stock based compensation expense	0	4	0	0	0	4
Balance at Mar. 31, 2020	100,205	176,431	294,553	11,392	(34,396)	548,185
Balance at Dec. 31, 2019	100,205	176,427	288,067	4,461	(30,903)	538,257
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income						38,638
Other comprehensive income, net of tax						7,076
Balance at Sep. 30, 2020	100,205	176,441	306,741	11,537	(34,396)	560,528
Balance at Mar. 31, 2020	100,205	176,431	294,553	11,392	(34,396)	548,185
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	11,254	0	0	11,254
Other comprehensive income, net of tax	0	0	0	544	0	544
Cash dividend declared	0	0	(6,568)	0	0	(6,568)
Stock based compensation expense	0	6	0	0	0	6
Balance at Jun. 30, 2020	100,205	176,437	299,239	11,936	(34,396)	553,421
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	14,071	0	0	14,071
Other comprehensive income, net of tax	0	0	0	(399)	0	(399)
Cash dividend declared	0	0	(6,569)	0	0	(6,569)
Stock based compensation expense	0	4	0	0	0	4
Balance at Sep. 30, 2020	$ 100,205	$ 176,441	$ 306,741	$ 11,537	$ (34,396)	$ 560,528