Consolidated Statements of Changes in Shareholders' Equity - USD ($)	Preferred Stock [Member]	Common Stock [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balances at Dec. 31, 2014	$ 0	$ 317,904,000	$ 10,218,000	$ 16,000	$ 328,138,000
Employee stock purchase plan		44,000			44,000
Dividend reinvestment plan		655,000			655,000
Stock option exercises		891,000			891,000
Stock grants to directors for retainer fees		403,000			403,000
Stock-based compensation expense		684,000			684,000
Share repurchase program		(15,762,000)			(15,762,000)
Cash dividends			(9,516,000)		(9,516,000)
Net income			27,020,000		27,020,000
Change in net unrealized gain/(loss) on securities available for sale, net of tax effect				1,247,000	1,247,000
Change in fair value of interest rate swap, net of tax effect				0	0
Balances at Dec. 31, 2015	0	304,819,000	27,722,000	1,263,000	333,804,000
Employee stock purchase plan		36,000			36,000
Dividend reinvestment plan		1,601,000			1,601,000
Stock option exercises		978,000			978,000
Stock grants to directors for retainer fees		327,000			327,000
Stock-based compensation expense		1,459,000			1,459,000
Share repurchase program		(3,732,000)			(3,732,000)
Cash dividends			(18,731,000)		(18,731,000)
Net income			31,913,000		31,913,000
Change in net unrealized gain/(loss) on securities available for sale, net of tax effect				(6,954,000)	(6,954,000)
Change in fair value of interest rate swap, net of tax effect				110,000	110,000
Balances at Dec. 31, 2016	0	305,488,000	40,904,000	(5,581,000)	340,811,000
Employee stock purchase plan		46,000			46,000
Dividend reinvestment plan		1,576,000			1,576,000
Stock option exercises		318,000			318,000
Stock grants to directors for retainer fees		363,000			363,000
Stock-based compensation expense		1,981,000			1,981,000
Cash dividends			(12,046,000)		(12,046,000)
Net income			31,274,000		31,274,000
Change in net unrealized gain/(loss) on securities available for sale, net of tax effect				1,493,000	1,493,000
Change in fair value of interest rate swap, net of tax effect				54,000	54,000
Reclassification of stranded tax effect related to available for sale securities resulting from Tax Cuts and Jobs Act			869,000	(869,000)	0
Balances at Dec. 31, 2017	$ 0	$ 309,772,000	$ 61,001,000	$ (4,903,000)	$ 365,870,000