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, 2016	$ 99,214	$ 171,425	$ 201,517	$ (6,251)	$ (33,219)	$ 432,686
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	35,783	0	0	35,783
Other comprehensive income (loss), net of tax	0	0	0	3,191	0	3,191
Cash dividend declared	0	0	(18,899)	0	0	(18,899)
Stock options exercised	348	1,510	0	0	0	1,858
Purchase of treasury stock	0	0	0	0	(1,683)	(1,683)
Sale of treasury stock	0	(337)	0	0	2,215	1,878
Stock based compensation expense	0	114	0	0	0	114
Balance at Sep. 30, 2017	99,562	172,712	218,401	(3,060)	(32,687)	454,928
Balance at Jun. 30, 2017				(3,593)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income						12,596
Other comprehensive income (loss), net of tax						533
Balance at Sep. 30, 2017	99,562	172,712	218,401	(3,060)	(32,687)	454,928
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	45,412	0	0	45,412
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	(9,848)	0	(9,848)
Cash dividend declared	0	0	(19,229)	0	0	(19,229)
Stock options exercised	177	1,082	0	0	0	1,259
Purchase of treasury stock	0	0	0	0	(718)	(718)
Sale of treasury stock	0	(121)	0	0	1,903	1,782
Stock based compensation expense	0	152	0	0	0	152
Balance at Sep. 30, 2018	100,175	176,764	246,965	(13,000)	(33,786)	477,118
Balance at Jun. 30, 2018				(9,796)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income						15,199
Other comprehensive income (loss), net of tax						(3,204)
Balance at Sep. 30, 2018	$ 100,175	$ 176,764	$ 246,965	$ (13,000)	$ (33,786)	$ 477,118