Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) $ in Thousands	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Surplus Cumulative Effect, Period of Adoption, Adjusted Balance	Surplus	Retained Earnings Effect of adoption	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Accumulated Other Comprehensive Loss Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Loss	Effect of adoption	Cumulative Effect, Period of Adoption, Adjusted Balance	Total
Balance at Dec. 31, 2018		$ 198		$ 352,093			$ 117,432		$ (781)		$ (15,112)			$ 453,830
Increase (Decrease) in Stockholders' Equity
Net income							37,483							37,483
Shares issued under the dividend reinvestment plan				643										643
Shares issued under the Employee Stock Purchase Plan				122										122
Purchase of treasury stock									(625)					(625)
Stock awards granted and distributed		1		(988)					987
Stock awards forfeited				479					(479)
Repurchase of surrendered stock from vesting of stock plans				(18)					(859)					(877)
Share based compensation expense				2,755										2,755
Cash dividend declared							(13,812)							(13,812)
Other comprehensive income (loss), net of deferred income taxes											6,884			6,884
Balance at Sep. 30, 2019		199		355,086			141,103		(1,757)		(8,228)			486,403
Balance at Jun. 30, 2019		199		353,729			131,811		(1,337)		(9,197)			475,205
Increase (Decrease) in Stockholders' Equity
Net income							13,903							13,903
Shares issued under the dividend reinvestment plan				224										224
Purchase of treasury stock									(304)					(304)
Stock awards granted and distributed				(33)					33
Stock awards forfeited				137					(137)
Repurchase of surrendered stock from vesting of stock plans									(12)					(12)
Share based compensation expense				907										907
Cash dividend declared							(4,611)							(4,611)
Other comprehensive income (loss), net of deferred income taxes											969			969
Balance at Sep. 30, 2019		199		355,086			141,103		(1,757)		(8,228)			486,403
Balance at Dec. 31, 2019	$ 199	199	$ 356,436	356,436	$ (1,473)	$ 149,230	150,703	$ (1,843)	(1,843)	$ (8,341)	(8,341)	$ (1,473)	$ 495,681	497,154
Increase (Decrease) in Stockholders' Equity
Net income							33,065							33,065
Shares issued under the dividend reinvestment plan				748										748
Shares issued under the Employee Stock Purchase Plan				128										128
Purchase of treasury stock									(4,633)					(4,633)
Stock awards granted and distributed				(2,880)					2,880
Stock awards forfeited				222					(222)
Repurchase of surrendered stock from vesting of stock plans				(36)					(1,014)					(1,050)
Share based compensation expense				3,086										3,086
Cash dividend declared							(14,396)							(14,396)
Other comprehensive income (loss), net of deferred income taxes											(408)			(408)
Balance at Sep. 30, 2020		199		357,704			167,899		(4,832)		(8,749)			512,221
Balance at Jun. 30, 2020		199		356,510			159,635		(4,961)		(8,762)			502,621
Increase (Decrease) in Stockholders' Equity
Net income							13,061							13,061
Shares issued under the dividend reinvestment plan				261										261
Shares issued under the Employee Stock Purchase Plan				122										122
Stock awards granted and distributed				(169)					169
Stock awards forfeited				20					(20)
Repurchase of surrendered stock from vesting of stock plans									(20)					(20)
Share based compensation expense				1,082										1,082
Cash dividend declared							(4,797)							(4,797)
Other comprehensive income (loss), net of deferred income taxes											13			13
Balance at Sep. 30, 2020		$ 199		$ 357,704			$ 167,899		$ (4,832)		$ (8,749)			$ 512,221