Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) $ in Thousands	Common Stock	Surplus	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss	Total
Balance at Dec. 31, 2018	$ 198	$ 352,093	$ 117,432	$ (781)	$ (15,112)	$ 453,830
Increase (Decrease) in Stockholders' Equity
Net income			23,580			23,580
Shares issued under the dividend reinvestment plan		419				419
Purchase of treasury stock				(321)		(321)
Stock awards granted and distributed	1	(955)		954
Stock awards forfeited		342		(342)
Repurchase of surrendered stock from vesting of stock plans		(18)		(847)		(865)
Share based compensation expense		1,848				1,848
Cash dividend declared			(9,201)			(9,201)
Other comprehensive income (loss), net of deferred income taxes					5,915	5,915
Balance at Jun. 30, 2019	199	353,729	131,811	(1,337)	(9,197)	475,205
Balance at Mar. 31, 2019	199	352,454	125,765	(786)	(12,629)	465,003
Increase (Decrease) in Stockholders' Equity
Net income			10,653			10,653
Shares issued under the dividend reinvestment plan		221				221
Purchase of treasury stock				(321)		(321)
Stock awards granted and distributed		(164)		164
Stock awards forfeited		342		(342)
Repurchase of surrendered stock from vesting of stock plans		(18)		(52)		(70)
Share based compensation expense		894				894
Cash dividend declared			(4,607)			(4,607)
Other comprehensive income (loss), net of deferred income taxes					3,432	3,432
Balance at Jun. 30, 2019	199	353,729	131,811	(1,337)	(9,197)	475,205
Increase (Decrease) in Stockholders' Equity
Cumulative change in accounting principle (Note 1)			(1,473)			(1,473)
Balance at January 1, 2020 (as adjusted for change in accounting principle)	199	356,436	149,230	(1,843)	(8,341)	495,681
Balance at Dec. 31, 2019	199	356,436	150,703	(1,843)	(8,341)	497,154
Increase (Decrease) in Stockholders' Equity
Net income			20,004			20,004
Shares issued under the dividend reinvestment plan		487				487
Shares issued under the Employee Stock Purchase Plan		128				128
Purchase of treasury stock				(4,633)		(4,633)
Stock awards granted and distributed		(2,711)		2,711
Stock awards forfeited		202		(202)
Repurchase of surrendered stock from vesting of stock plans		(36)		(994)		(1,030)
Share based compensation expense		2,004				2,004
Cash dividend declared			(9,599)			(9,599)
Other comprehensive income (loss), net of deferred income taxes					(421)	(421)
Balance at Jun. 30, 2020	199	356,510	159,635	(4,961)	(8,762)	502,621
Balance at Mar. 31, 2020	199	355,014	153,766	(4,800)	(10,926)	493,253
Increase (Decrease) in Stockholders' Equity
Net income			10,656			10,656
Shares issued under the dividend reinvestment plan		247				247
Shares issued under the Employee Stock Purchase Plan		128				128
Stock awards granted and distributed		(43)		43
Stock awards forfeited		182		(182)
Repurchase of surrendered stock from vesting of stock plans				(22)		(22)
Share based compensation expense		982				982
Cash dividend declared			(4,787)			(4,787)
Other comprehensive income (loss), net of deferred income taxes					2,164	2,164
Balance at Jun. 30, 2020	$ 199	$ 356,510	$ 159,635	$ (4,961)	$ (8,762)	$ 502,621