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, 2017	$ 99,998	$ 175,651	$ 219,436	$ (1,806)	$ (34,971)	$ 458,308
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	14,808	0	0	14,808
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect	0	0	1,346	(1,346)	0	0
Other comprehensive income (loss), net of tax	0	0	0	(5,338)	0	(5,338)
Cash dividend declared	0	0	(6,323)	0	0	(6,323)
Stock options exercised	4	16	0	0	0	20
Sale of treasury stock	0	(21)	0	0	615	594
Stock based compensation expense	0	28	0	0	0	28
Balance at Mar. 31, 2018	100,002	175,674	229,267	(8,490)	(34,356)	462,097
Balance at Dec. 31, 2017	99,998	175,651	219,436	(1,806)	(34,971)	458,308
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income						45,412
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect				(1,346)
Other comprehensive income (loss), net of tax						(9,848)
Balance at Sep. 30, 2018	100,175	176,764	246,965	(13,000)	(33,786)	477,118
Balance at Mar. 31, 2018	100,002	175,674	229,267	(8,490)	(34,356)	462,097
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	15,405	0	0	15,405
Other comprehensive income (loss), net of tax	0	0	0	(1,306)	0	(1,306)
Cash dividend declared	0	0	(6,330)	0	0	(6,330)
Stock options exercised	91	592	0	0	0	683
Purchase of treasury stock	0	0	0	0	(379)	(379)
Sale of treasury stock	0	(72)	0	0	668	596
Stock based compensation expense	0	49	0	0	0	49
Balance at Jun. 30, 2018	100,093	176,243	238,342	(9,796)	(34,067)	470,815
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	15,199	0	0	15,199
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect				0
Other comprehensive income (loss), net of tax	0	0	0	(3,204)	0	(3,204)
Cash dividend declared	0	0	(6,576)	0	0	(6,576)
Stock options exercised	82	474	0	0	0	556
Purchase of treasury stock	0	0	0	0	(339)	(339)
Sale of treasury stock	0	(28)	0	0	620	592
Stock based compensation expense	0	75	0	0	0	75
Balance at Sep. 30, 2018	100,175	176,764	246,965	(13,000)	(33,786)	477,118
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 (loss), net of tax	0	0	0	3,298	0	3,298
Cash dividend declared	0	0	(6,591)	0	0	(6,591)
Stock options exercised	5	30	0	0	0	35
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 (loss), 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 (loss), 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 (loss), net of tax	0	0	0	1,703	0	1,703
Cash dividend declared	0	0	(6,599)	0	0	(6,599)
Stock options exercised	20	98	0	0	0	118
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